1. Exe File Creator

Unlock/Softmod/Hack your Wii for Free! Required: An SD card (Best results are with non-SDHC cards, SDHC will only work on 4.0 or above) or USB device formatted to FAT(32). I modified ifrit05's 'Dolphin SDCard Maker v1.0.bat' to include a 4GB. Find 'start mksdcard.exe -l SDCard XM sd.raw' where 'X' could be any. Sygic product code keygen.

DRGN

Technowizard

You'll find a pretty large collection of links and resources below. But also check out the separate sub-forums if you're interested in a particular subject. The stickied threads in each of the sub-forums also contain good starting points and a lot of info. Chances are high that the information you're looking for is either already available in one of these places, or within a thread from someone asking the same question. Which brings me to my last point, don't forget about the site's search feature, it's really useful!

20XX Tournament Edition - created by Dan Salvato - This is a really awesome hack that not only allows you to augment your Melee experience with lots of cool and convenient features (such as stage-striking, all default tournament settings, replay saving, the Taunt Battle game mode, and more), but it allows you to do so with any unmodified console using nothing but a memory card and a special save file! Amazing! You'll want to just go watch the trailer now. Please note that this and achilles' 20XX Training Hack Pack below are actually separate projects. (So while they may use some features based on the same code, they don't officially provide support for or tie-in to one another.) Watch the trailer and learn more here.
The 20XX Melee Training Hack Pack - created by achilles - Experience the Melee hacking sensation that has the whole world burning for more! Ok, maybe I’m exaggerating a little bit, but not by much. This thing really is pretty amazing. And although it says “Training Hack Pack”, it’s not actually just for training; it’s got a ton of great new stuff to play with: modified and new stages, character costumes, game controls, custom music playlist control, new game modes, and more. The benefit of this mod over 20XXTE is that it has more content, however the downside is that it is only playable on hacked consoles. Besides normal, unmodified characters, it also has the SD Remix cast built-in, as well as PAL variations. Check out the feature list in the OP to see all the stuff you don’t know how you lived without.
Melee: SD Remix - created by VietGreek, Ripple, & standardtoaster - This is a carefully thought-out mod that aims to balance the cast through only conservative character buffs. In fact, this means that the higher-tier characters are barely, if at all, touched. As stated in the OP, SD Remix intends to improve the weaker characters to viable levels by: addressing their crippling weaknesses; providing them stronger offensive and defensive options in line with the stronger cast members; maintaining each character's archetype and niche. The mod does a great job to add in a bit of balance without changing the mechanics, feel, or flow of Melee. Full changelists are available.
Melee Netplay Community Build (MNCB) - organized by Myougi - This is a build specifically geared towards playing Melee online. Useful since it provides a shared standard, meaning that online games will be the same and don't desync, while addressing some of the remaining Netplay issues and adding some nice extra content.
Tata Hack Pack - created by tatatat0 - This is a full game mod that changes characters, game mechanics, UI, stages, and more. Characters are balanced through buffs, but with much more aggressive changes than in SD Remix. And the game has been modified to be faster, with new advanced techniques and drastically remastered character archetypes. It even has new moves and abilities. Try it out for a fresh twist!
The Falcon Ditto - created by CaptainJazz - Do you like Falcon? Good. Have some more! And..then, even more. (Especially recommended if you also like saxophones and smooth customers.)

How-To: Gamecube Games on Your Wii U (or newer Wii) - written by Doqtor Kirby - Title says it all.
SSB: Melee TAS. Getting Started - written by Nakamaru - Perfect DK vs. Perfect Samus? Who would win!? Find out here! Er, at least, find out how to make a match along those lines. Or whatever else your imagination can cook up.
Burning a Texture Hacked Copy of Melee - written by MuraRengan - Most people play their hacked copy of smash via SD card or external hard drive and a hacked console rather than disc (so you don't have to re-burn your disc after making one small change). But if you'd like to create a new disc instead, for example to play on an unmodified console, then this is the guide for you.
The Definitive Guide to Exploring File Formats (TDGTEFF) - written by Watto and Mike Zuurman - An extensive resource that covers the basic fundamentals of hex editing, and then goes deep into how game archive formats are created and how to reverse-engineer them.

The Crazy Mod and wParam's SSBM Website - created by wParam - This, ladies and gentlemen, is the secret behind the Memory Card hack method, used to encapsulate a collection of code mods in your standard, humble GC memory card (such as seen with 20XXTE). It's actually ancient technology, before the Dolphin Age, that was only recently found potential realized. It works by using an exploit in Melee's custom nametag feature, allowing custom code to be loaded and injected into the game. Perhaps the most beautiful thing about this method of injecting codes is to be able to use and share hacks to completely unmodified consoles. And you can then even copy the hacks to another memory card by using the GC or Wii's standard memory card manager. Amazing!
YAGCD, 'Yet Another GameCube Discussion' - Contains a ton of very in-depth GameCube hardware & software specs and documentation: available RAM, CPU instruction set, boot instructions, file format descriptions, and much more.
Melee dat format… - created by revel8n - DAT files are the primary type of data file that Melee uses to store stages, characters, trophies, menus, images, fonts, and more. This thread walks you through the hierarchy of how they're made up. Needless to say, this is for advanced hacking purposes.
DAT File Research - created by zankyou - The OP does not have quite so much information as the resource above and below, but you can find much experimentation and info by looking through the thread.
HAL DAT Format (Mario Kart Wiki page) - by Tcll, Achilles, and others - Another great resource on the make-up of .dat files. It's in a little different format from above, and there isn't as much description here as you go, but still very useful. And you might find additional info with this one, especially considering that this page is still actively developed.
Post by GodFed on Relocation Tables - Relocation Tables are a specific data structure within .dat files. revel8n talks about these in his thread above, but reading this too can give you another valuable point of view, which may help connect the dots.

The Melee Toolbox - created by DoctorKirby - This is a pack that contains a vast majority of all of the other programs for Melee modding found on these forums. Useful as a quick one-stop download, plus it has a menu system that can be used to open the programs within. Also, there is it's sister project, Project Neko. It is foretold that at some point these two projects will merge into one, presumably by fusion dance.
Dolphin - The great emulator used to run GameCube and Wii games on PC. Even if you have this already, you may want to visit their download page anyway, considering how often new, improved versions are released. It also has a ton of built-in features useful to game hacking, including for reverse-engineering ASM code and game functions in RAM. Build 5.0-3977 (mirror) is currently recommended for this, as it is confirmed to be able to break (pause) the game when specific points in RAM are read or written to, rather than only on code execution. (The DebugFast Dolphin build is no longer required. Details on this can be found here.) Program usefulness level: Invaluable.
DAT Texture Wizard - created by DRGN - Although initially created as a tool for importing/exporting textures from Melee, it has evolved to be a fully-featured ISO/GCM disc management tool as well. It can import/export files from discs (even those larger than the originals), add/remove files/folders, rebuild in-place or build a new disc from files, modify disc details (such as game ID/titles, etc.), import/export the game banner, and more! Amazing! These disc functionalities also work for other GameCube games. As an alternative,
GCRebuilder can also perform some of these features. (.Net Framework 3.5 or higher is required for it run; you can find that free from Microsoft here.) However, DTW is certainly recommended for Melee, as it also has a number of special-purpose features tailored to help reduce the chance of user-error when adding mods or costumes to your game.
GIMP - A free alternative to Photoshop. It can do most, if not all, that Photoshop can, although it works a little bit differently and usage may seem peculiar in some ways to newcomers. It can also do some things that Photoshop cannot, including easy alpha layer manipulation, easy plug-in creation, and even the ability to work from command line (Hello, Automation!). You can use pretty much any image editor (as long as it can work with palettes if you’re doing a paletted texture) to create/edit your textures though (but I’d still recommend something better than Paint).
HxD - There are many hex editors out there, offering the same basic functionality. So choosing one of these is typically personal preference, usually based on the extra features that make certain tasks quicker & easier. (Comparisons of a few.) HxD is a good, free hex editor that many of us use to open up .dat, .usd, or other files for manual editing or searching. It's very useful, but you may not need this depending on what you're doing. Another one you may want to consider is 010 Editor. Although the latter is not free, it does have the additional feature of templates and bookmarks, which can highlight different areas of the file as you're viewing it, to make them easier to read. A template for some of Melee's files was created by revel8n, and can be found here.
HexEdit - I haven't played with this yet, but it looks like it has some good features. It also supports templates (for automatic highlighting of certain sections). Tcll offers a template you can try here.
SSBM: Version Converter/Patcher - created by chain-ace - This allows you to convert an ISO of one version of Melee to another (e.g. v1.00 to 1.02). Quite useful if you cannot find the other version, but the program sometimes doesn't work on some systems. See here for how to check your [vanilla/unmodified] game version using an MD5 hash.

Melee Files and their Purpose - Open an ISO and what do you find? The files explained in this sheet.
List of regional version differences - There are several different release versions of SSBM, with some slight differences in menus, character weights, some attacks, game mechanics, glitches, and more. See here for how to check your [unmodified] game version using an MD5 hash.
The Cutting Room Floor on SSBM - Lots of tidbits of unused or unfinished pieces of the game. An interesting browse, and maybe even be a little insightful in how the game does a few things.

The Complete Music (and Sound) Compendium - created by DoctorKirby - A help, support, and sharing thread to talk about audio hacking, or publish any cool new SSM hacks. The posts directly following the OP also contain a ton of songs and other audio hacks you can download, ready to go right into your ISO!
Brawl Custom Music (.com) - 'What?? Brawl in my Melee?!' Uhh, well, not quite. It's not actually a guide either, but this is a great resource for lots of music (in the form of HPS files) that you can add directly into your [Melee] game. Just choose 'Melee' in the 'Download for' dropdown.
Audio Melee Hacks - Music and Sound Requests - created by Tichinde925 - If the guides below are too much, you can make your requests here. Or even better, create some new hacks that others have made requests for! There are also more songs and other audio hacks in the OP for download.
SSM Audio Hacking Guide - written by GodFed - SSM files are the audio files that hold the short audio clips in the game. The character voices, the announcer voice, all the blips, clicks, and clangs, etc. are in SSM files. This guide goes over how to change these.
HPS Audio Hacking Guide - written by GodFed - This post delves into the details of the HPS format, explaining the structure of the files. Useful for more advanced or specific hacking needs. If you don't need all the details, you can go strait to the programs below, which will handle these things for you.
DSP Visual Tutorial - written by Tichinde925 - A picture's worth a thousand words. So, by definition, this visual guide is worth 15,000 words of tutorial, or something. Tichinde uses HxD, the free hex editor, to demonstrate how to manually hex SSM files. Check it out.

20XX Music Manager - created by Deconimus - This is a program specifically designed to add/remove songs from your copy of 20XX (Hack Pack v4.0+, not TE). Using this is more convenient than simply replacing the files in your ISO manually, because it also updates tables within 20XX that contain the song names of the respective files. There is also a nice explanation here on how to use 20XX's custom playlists feature, which is actually as awesome as it is easy to use.
dsp2hps - written by jmlee337 - This is a command line application that converts many different audio formats to HPS (Melee's music file format), so you can add your own custom music to your game. This program also allows you to set loop points in your audio, and solves many of the old issues that used to plague the HPS creation process. Amazing! GUIs for it are available below.
Melee Audio Converter - created by Deconimus - This is a GUI frontend for the dsp2hps application above. Requires Java.
d2hgui - created by Doctor Kirby - An alternative GUI for dsp2hps.
WinAmp with the in_cube plugin - Used as a high quality player for .hps music files. Useful to test how your audio will sound in-game before adding it to your ISO.
Audacity - A feature rich tool for audio editing. Allows for adjusting sample rates, both left and right channels, and much more.
Audacity with GameCube DSPADPCM support - A fork of Audacity on GitHub that supports importing/exporting audio formats such as DSP and PCM files. Very useful!
SSBM Audio Tools - A download pack containing several audio tools, including:

HPSWAV.bat - created by bilbaggins - A batch file to convert HPS files to WAV format (uses vgmstream test.exe).
MeleeSsm - created by GenesisFan64 - For converting/working with sound effect (.ssm) files.
vgmstream - created by hcs - converts HPS and DSP to WAV.
Halley's Comet Software - Used for working with many video game audio formats.

A post on normalizing and amplifying music volume - by Myougi - Useful so your music doesn't crackle, pop, or drown out the game's other sounds.
HPS info dumps: Part 1 and Part 2 - written by Eternal Yoshi - A whole bunch of info on Melee's looping songs, including for each song: sample rate, channels, loop start & end, total stream samples, encoding, metadata, layout, samples to play, offset info, loop points in song, total audio blocks, and total time.

Melee Syntax and Guide. Let's make super characters! - created by Itaru - This is an interesting thread that talks about editing special effects, hitbox/attack commands, and how they're placed in .DAT file data structures. If nothing else, at least check out the amazing, insane video found there.
Cosmetic Hitbox Element Guide - written by Lanceinthepants - You can do some cool stuff with this guide, such as change moves to have electric, darkness, fire, or other effects.
Changing Color Effects in Melee! - created by Lanceinthepants - Many texture types are standard images which include their own colors. But some (types _0 through _3) are actually given their color by other code in the game as they’re rendered. This guide explains how to change the hex that defines these colors, for various effects throughout the game. Look through the thread and you can find the offsets (locations in files) to tons of things.

Effect Changer - created by CeLL - A simple program built to help you change the colors of different effects. And if you need it, here's a 32 bit version.
Laser Changer - created by CeLL - Program specifically made to easily change the colors of Fox and Falco’s lasers.

Side-B phantom hacks - written by nube - How to change the color of Falco and Fox's Side B.
Texture Hacking Effects - written by DRGN - Some examples of what you can do with hacking effects. There's also a spreadsheet that gives offsets and descriptions to findings in EfCoData.dat (Effects Common Data), a file that contains effects common to the cast. Neat effects pack (Onomatopoeia hack) included in the second post.

Assembly Guides, Resources, & Q/A - organized by Dan Salvato - This thread contains lots of great resources for assembly programming. Assembly (ASM) code is what you get when you take the game's hexadecimal code that you see in the files and decode it. It's the primary language that Melee coding is done in. Although, programming for Melee in C has begun to poke its head up too. This thread is a must-visit if you want to get into this.
Let's learn assembly - created by Hylian - A thread dedicated to learning assembly code (ASM), which can then be converted to hex (using the converter found in this section's Programs list) to create Gecko codes or DOL mods. There are even more great resources to be found throughout this thread; just start reading through.
Melee Gecko Codes + Guide and Discussion - created by Dan Salvato, now updated by achilles - Gecko is a codehandler which can inject custom code into the game (think of it like a cheats engine). It can translate ASM (assembly) instructions to hex, and even has its own set of instructions that would normally take many more lines of ASM to do. This thread shows you how to apply Gecko codes, and supplies many as well. There are tons of cool codes you can implement into your game; from having everything unlocked with standard tournament settings applied by default, or stage-striking functionality a la PM, to for-fun codes like Mewtwo's 'Always full Shadow Ball' or infinite float for Peach. Great place to discuss/share new codes of course, too.
The DOL mod topic - created by SypherPheonix - DOL mods are essentially Gecko codes written in ASM (assembly), turned into hex, and then put into the game's Start.dol file (a.k.a. main.dol), which is then included into the ISO. This basically makes the ISO a stand-alone hack.
Intro to Wii Game Modding - created by the legen, I mean, Dan Salvato - This is THE definitive intro for all Melee coding and Debug Dolphin use. It's a long and detailed video tutorial on how the hacking process works and how you can go about figuring out new hacks. More than just a good watch, this is basically required homework, and you'll be just about immediately directed to this if you start posting questions on creating code. It's that good.
New Melee Syntax school (Writing Character Commands) - written by Itaru - This explains the format for character commands (things like graphics, sound effects, hitboxes, and more, relating to characters' moves) and how to write them. Check out the promotional video in the OP for some examples of what can be done.
Animation/Subaction Swapping - written by phish-it - Talks about swapping an animation from one move to another. Bowser dair turned into item-throw animation? Good stuff.
Physics, Input, etc Display in Develop Mode - created by Magus - Codes to display various character and controller data in real time in Debug Mode. Very useful for analyzing mechanics and other RnD work.
How to Change the Default Character Names that Appear on the Character Select Portraits - written by achilles - Super long, descriptive title is descriptive (and super long). Notably, it also contains NametagHackGuide.xlsx, which is a useful spreadsheet to look up the hex codes that pertain to letters and numbers. This allows you to search for, and change, the strings of text found in the game. So this actually serves as a guide for changing more than what the title says. Perfect for finishing off your new game mechanics hack: text display to the player so they know what's going on.
Animation Hacking Documentation - written by Itaru - This is really along a subject that should have it's own section, if only we knew more things to put in such a section.. Hopefully this thread can serve as a base to some people who want to help figure animations out.
Animation Compendium - A collection of various resources on animation documentation and hacking.

SSBM RAM Addresses (v1.02) - created by Dan Salvato, achilles, Massive, and more - As the name says, this spreadsheet describes the data in RAM as the game runs. A short description such as this can’t even begin to explain how to use this, how useful this is, or its potential for writing game code, so you’ll have some reading to do before you can really sink your teeth into this. But don’t worry, I’ll just keep it for you here until you come back.
Community Symbol Map - started by Absolome - From the beginning, Melee's source code has been kept locked away in the Vaults of HAL, which makes reverse engineering the game rather difficult. Any attempt to decompile the raw hex game code, the 'DOL', necessarily loses all function and variable names (called symbols), meaning it is still difficult for a human to read and understand. The Community Symbol Map is a project to address this; by using Dolphin's debugger, and a .map file (the heart of this project) composed of the collective info and R&D of various users who have named individual functions. It helps a ton when stepping through the game's code to see what's going on. Check the project's GitHub page if you want to make sure you get the latest version.
Melee Mechanics by Kadano - This is a YouTube channel full of very well-done, highly in-depth explanations of Melee’s internal mechanics. Admittedly, not directly hacking-related, but I think it’s useful to learn more about how exactly the mechanics of the game work in order to figure out how to reverse-engineer and change them.
wParam's notes - written by wParam - A very old, yet recent find. Some of the notes are fragmented and a bit hard to decipher. As SinsOfApathy said, it's sort of the 'Book of Shadows' in Melee spellcraft. Basically, it's more offsets and notes on the game's RAM and the creation of The Crazy Mod, and is therefore similar in its usefulness as the RAM addresses spreadsheet above this, but with additional info.
SSBM Hitboxes (NTSC v1.00).xlsx - created by ? - This is a huge spreadsheet containing information on every hitbox in the game. Some say it must have been made programmatically (by going through the dat files), others say some legend crafted it with his bare hands.. There's also Stratocaster's mirror of it here, which is viewable without having to download it.
Character Attributes - created by ? - A table on each character's various physical attributes and stats: jump height, landing lag, weight, friction, running speed, and more. This data is found in each of the characters' Pl**.dat file. The point in the file where the data begins varies by character, but you can find where the data starts for each character here.
Melee Character Files - created by ? - Here are the many .dat files for each character for every version of the game, nicely organized into subfolders for moveset, textures, effect, and DViWait files. Great for a program database. Have fun not having to extract all of this yourself.

Melee Code Manager - Easily Add Mods to Your Game! - created by DRGN - This program provides a very easy way to add mods to your game, manage what codes you have installed, and even change the game's default settings! It also provides powerful features for code developers, allowing for automatic management of DOL Mods (code overwrites and injections that integrate directly with the game's original code), which are much more efficient than using Gecko codes (although those are supported by this program too!). All codes are fully integrated into the game disc rather than an external file like a .gct or .ini. Other tools are also built-in, including ASM<->Hex, various number converters (e.g. decimal to/from hex and/or floats), DOL Offset and RAM Address converters, ASCII Text<->Hex, and more. Marvelous!
Crazy Hand - created by Ampers, darkside, and Tater - This is a character editing program to allow you to edit things such as character attributes, hitboxes, subactions, and more. This is homage to the predecessor program Master Hand, which was an early RnD tool for the development of Project M, to read (but only read) Melee's character and move properties. Crazy Hand can already do a lot, and is still in development, with new features continually being added. You can also find a ton of information on how the game stores and reads hitboxes, subactions, action events, and more within the first couple of pages of the thread.
Tata's Interrupt changer - created by tatatat0 - This is a program to manipulate the mechanics of non-character specific subactions, which was once only possible by wizards. You can do anything from making dash-attack teleport, or DACUS, to changing all kinds of move physics.
ASM <> WiiRd Converter (Gecko.NET download) - This is used to convert Gecko codes to ASM, or ASM to Gecko. Conveniently, this particular download actually comes with the required components (how nice!).
IEEE 754 Converter - Many things in Melee are stored as floating-point numbers, as hex, when you really just want to know the decimal version. This tool converts between floating-point, hex, and decimal.

Perfect 16:9 Widescreen Support (with a nice little demo vid) - code created by Dan Salvato - Title says it all. Although, you might want to read a bit into the thread, as newer codes to fix a few issues have been posted.
Fixed Toggleable Alternate Character Textures - written by zankyou - This is a code that allows you to double the number of alternate costumes you can have in your game for each character!
On hitboxes - written by Magus - How hitboxes are described in Melee's game files; how data such as damage, angle, and knockback is stored and how to find it.
On throw commands - written by Magus - Basically the same as above, but with throws.
On special subactions - written by Magus - A short note on special moves' subactions, and the currently known commands.
On shieldstun/hitstun - written by Magus - Some short notes.

Stage Hacking: NEW Research + Discussion - created by Milun - Read through this thread and you'll find tons of info on stage model hacking. Learn how to edit camera movement ranges, blastzones, stage objects (models & collisions), platform positions & properties, ledges, and more! This thread and the one below are basically required reading for stage hacking.
Melee Stage Hacking Documentation (WARNING: Not User-Friendly) - created by Dan Salvato - As the disclaimer in the OP says, this is meant for experienced hackers. But it doesn’t take much before you start realizing that this is some great documentation here. It basically explains how the data structures are organized in stage .DAT files, and how the game (or we) can follow the hierarchy to different levels and objects.
Dat File Research - create by zankyou - This thread talks more about stage dat structures, and various flags/properties of the structures.
SSBM Protocole stage.dat - created by Yax - A Google Docs spreadsheet that, like above, goes over what a typical stage .DAT file looks like and how its data structures are organized. It’s organized into easy-to-read sections, with some added notes to give you another perspective to accompany the one you gain from Dan’s docs.
Removing Parts of a Model - written by Milun - This guide does just what it says on the box, showing you how to remove parts of both character and stage models. It also goes on to talk about the Material Structure in DAT files, and how they affect the shininess, darkness, and other properties of textures. Very useful!

Official Texture Hack Thread - created by Veggies - Although this is mostly character skin (texture) related hacks, there are lot of custom stages included here as well if you’re looking for those.
Hacked Stage Catalog (spreadsheet repository) - created by Pac-Man Vs. - A spreadsheet containing a list of hacked stages, including links for downloads, different tweaks of different stages, and video demos. It's a little outdated, but still includes variations of stage hacks that aren’t included in the thread above.

VertConvert - created by Milun - Converts model objects to hex, which you can then paste into the proper places in your dat. Still requires some knowledge and legwork to use, but gives you some nice power.
Universal Model Converter - created by Tcll

Exe File Creator

- This is still a work-in-progress, which Tcll has been quite hard at work at. It aims to be able to convert a 3D model of any format to any other format. Eventually he even plans to work out animations too. I believe so far it does work for Trophies.
SSBM Ty Model Converter - created by jahra!n - One of the first recorded attempts at creating a model viewer. It's incomplete, and I believe only [partially] works on trophies. Read through the thread a bit and you'll find a few more scripts that may help with the use of this program.

Tcll's 1st emutalk model viewer thread - Old and no longer used, but may contain information useful to those trying to get a better understanding of certain aspects of models.
Tcll's 2nd emutalk model viewer thread - Same as above.

Official Texture Hack Thread - created by Veggies - The OP is a massive repository for all existing character and stage skins (as well as a few other miscellaneous texture hacks). This is the place to post new designs as well. If you find a broken link in there or don't find what you're looking for, leave a post!
The Old Melee Texture Hacks Thread - created by D1 - The old version of above. Ideally all the textures found here should be in the above thread, but until that's confirmed I'll leave this here in case you'd like to look through it. (If you find some skins in here that aren't in the Official Texture Hack Thread, lets get them moved over. Or let me know if that's already happened and I'll remove this.)
SSBM: HQ (texture) - created by Steelia - As a side project, Steelia decided to upgrade the Melee cast's textures. The results are quite an improvement! (You can see his video showcases here.)For most characters this is only for their default color, but for a few he has redone their original alternate costumes too. He also has a couple stages upgraded and some original alternate costumes near the bottom of the OP (including a Wolf texture complete with model-hacked eyepatch!). Steelia also has these and more of his uploaded here, though you should already be able to find all of those either in Steelia's SSBM HQ thread or the Official Texture Hack Thread.
The Complete CSP Compendium - written by Doqtor Kirby - Character Select Portraits (CSPs) are the character portraits that show up along the bottom of the Character Select Screen (CSS) after you've chosen your character and color. These aren't linked to the character models or the model's textures in any way; they're just images, which means that if you want your CSPs to reflect the hacked character costumes you have, you'll need to replace these as well. There are currently waaay more character skins than CSPs, mostly because not everyone knows how to make them, but you can find a lot here.
Some Custom CSPs - created by ChinesePanda - And by 'some' it means a lot. Hopefully will eventually will be merged with the thread above.
IllusoryLabyrinth - maintained by Steelia - A very old repository of ancient texture hacks. Some of these were never brought over to the boards, so you might be able to finally find a costume that has been, up until now, illusive.. Direct downloads of costumes there can be found here. You can also see demos of about all of those textures in the '__ Textures [SSBM]' videos here.

The Guide to Hack Any Texture - written by DRGN - A complete guide that covers the full process of opening up your GCN/ISO game image, editing your chosen texture (of any texture/image type), and putting it back into the game. Should be easy to follow for even complete beginners; it basically contains everything you need to know as well as lots of good resources. Also includes advanced hacking techniques at the end of the guide. This is a good place to post questions or ask for help on the process.
Changing Color Effects in Melee! - created by Lanceinthepants - Many texture types are standard images which include their own colors. But some (types _0 through _3) are actually given their color by other code in the game as they’re rendered. This guide explains how to change the hex that defines these colors, for various effects throughout the game. Look through the thread and you can find the offsets (locations in files) to tons of things.
Changing Menu Text - written by Jorgasms - A lot of the text in the game is actually done using images/textures, but some of it you can change by simple hex editing. Jorgasms shows you how it's done.
[Download + Tutorial] 20XX Costume Previews - written by UnclePunch - This is a full download of costumes (many of which are not in the default 20XXHP distribution) and Character Select Portraits (a.k.a CSPs; the character image that shows you the character and color you've selected) for the CSS. AND, there is a detailed guide here on how to make your own CSPs for any other costumes you may have!
Explanations of the TPL file format:


DAT Texture Wizard (DTW) - created by DRGN - This is a complete texture hacking tool for any of the game's files that contain textures (characters, stages, menus, etc.). It allows exporting/importing of files from/into your ISO or GCM disc image file as well, and even has a method for large batch texture importing, spanning any number of files in your game all at once. Accepts both TPLs and PNGs of any texture type. It also includes Structural Analysis of character files, allowing you to change texture & object properties such as shininess, or coloring of things that aren't textured. Using the Character Color Converter tab, the program can also convert costume files meant for a specific color slot to be able to go over a different color slot (for example, a red costume to replace a blue one). Simply one of those must-have programs for Melee Hacking.
The Tri-CSP Creator - created by DRGN - CSP stands for 'Character Select Portrait'; it's the preview image you see on the character select screen (CSS) when you pick a character so you know what they look like. Have you seen the CSPs in the 20XXHP that show all of the costumes, simultaneously the L/R alts as well as the default costume? Those are called Tri-CSPs, and this program creates them. The OP describes how you can make new CSP images using any character costumes, so once you've downloaded and imported new costumes into your game, you can update the preview images seen in-game to match so people know what they're selecting! Not only that, but these are the fabled 'pixel-perfect' kind, where the poses and positions of the characters are identical between the different CSPs. That makes for a very polished look as you cycle through the colors in-game, since the colors will be the only thing to change. You can also use the methods described in this thread to create normal (mono-?), pixel-perfect CSPs.
PNG to-from TPL - created by DRGN - This is for converting textures between PNG and TPL file formats, in either direction. This is a fairly new script that’s not only faster and easier for a single texture than the old methods, but can easily do large batches of images all at once. You might not need this though since DAT Texture Wizard can convert textures as well, but there are still a few good uses for having a TPL copy of your texture. Note that if you open it in a text editor, you’ll find a few options in the beginning of the file that you can change.
Texture Finder - created by CeLL - Used to search for textures in DAT/USD files. You can even tell it to search for multiple images among multiple different dat files. Though seek times can add up, so you may want to limit your searches to a few at a time. Currently supports texture types _0 through _9.
KirbyTool - created by libertyernie - Another tool for automatically writing textures into DAT/USD files. Support is limited to only a few characters however. Full instructions are available in the download.

Fixed Toggleable Alternate Character Textures - written by zankyou - This is a code that allows you to double the number of alternate costumes you can have in your game for each character!
Steelia's Texture Uploads - created by Steelia - Uploads of characters' and stages' original textures. Also contains a list of the file names for character color files (also available in DTW).
Texture previews (Steelia's YouTube) - If you're new to the concept of texture hacking, you can check out what some of the textures and stage hacks really look like live, in-game. Most of the videos here are really quite old though, so I wouldn't look to the guides you might find there to learn how to do stuff, since there are better methods nowadays (look around in this post for up-to-date guides). You can find the textures for download in the threads listed under Discussions and Resources above.
Old texture hacking methods: CMPR Texture Hacking (for _14 type textures), written by GodFed, and a guide on CI4/CI8 Texture Hacking (types _8 and _9, which are textures with palettes), written by Steelia. Though these are provided here kinda just-in-case and shouldn't be needed anymore. For converting textures between TPL and PNG, it is much easier to use the 'PNG to-from TPL' script listed with the programs above.
Creating CSS Closed Ports in GIMP - written by DRGN - This explains how to edit the closed port doors on the Character Select Screen (CSS). You know, the port/door things that are closed when you first go to the screen, but open when you move your cursor up to select a character. Those things. There's also more info, on how those textures deal with transparency, here.

MTH Video Hacking Guide - written by GodFed - MTH files are generic video files. The opening video falls into this category. This guide goes over the theory behind hacking them.
Opening Movie Hacking For Dummies - written by Cobalt - This explains how to hack the opening video (the video that plays automatically when you start the game). It's well organized and clear and awesome, so go check it out!

thpplay.exe - by Thakis - Player for THP and MTH files.
mth_make.exe - created by GodFed - Makes MTH files from JPG sequences.

Example Opening Movie - created by SleepyK - Just an example. No download link (though if someone has this I could add it here).
1-frame MTH video file - Use this to replace all those long video files in your ISO, which can free up a lot of space and allow you to fit in more custom content!

That's all for now. ^_^ Please let me know if you think a resource on here doesn't need to be here because it's redundant or outdated, or if you think there's something useful that you don't see here.