It's my intention to make this thread your one-stop for finding tools, resources, and information to get you started on making your very own ROM hacks and actual games. This is a work in progress, but hopefully it'll be comprehensive eventually. If anyone knows of anything they feel should be added here, feel free to PM me about it.

ROM Hacking Utilities

Nameless Sprite Editor - This is a good resource for locating and altering the sprites in GBA games. It's geared towards Pokemon with plugins that easily locate trainer card sprites and back sprites. I've not personally be able to figure out how to edit map sprites in the newest version, so if you need to do that and are as stumped as I am, try NSE classic.

Advanced Palette Editor - This tool is very useful in fixing palettes in GBA ROMs. If you modify just the sprite pixels itself, the colors won't change, no matter what colors you use unless the program you are using also edits the palette. This can cause your sprite to look really weird with random coloring. Using APE, you can directly alter the palette sets so that the colors will be correct.

unLZ-GBA - Some GBA ROMs use a LZ compression, making it impossible to see the graphics with a regular tile editor. If your other utilities for getting graphics fail you, try this and see if you can view them that way.

Free Space Finder - This is an advanced tool that most novices shouldn't be messing with... but if you're ready to become an intermediate hacker, you'll need it. What it does is find free space available in your ROM so you can re-point data. If you don't know what re-pointing is, chances are, you're not ready for this.

Sappy - This is a program that lets you replace songs in GBA games. You can do this with midi files if you properly convert them to .s files with Mid2Agb (should be included, but if it's not, someone let me know).

TiledGGD - For games that aren't GBA Pokemon, you may need to use more generic tools for altering the graphics. These are known as tile editors. There are more of them than I care to count, but this is one of my favorite because it's easy to use and easy to direct-edit within the program itself.

Tile Molester Alternate - Strange name, but a popular tile editor among the Pokemon hacking community.

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Game Makers & Resources

RPG Maker series - The RPG Maker website where you can download trials or purchase your desired version of RPG Maker software. They also have IG Maker, which is a generic game maker program designed to create stuff that's not RPG (Platformers, shooters, etc). Also be advised that for older versions of RPG Maker (XP and prior), you can actually just get them from the Control Panel

Fighting Game Maker 2nd Round - By the company that brought your RPG Maker (Enterbrain), this is an engine designed to make 2D fighting games. I like it a lot better than MUGEN.

Game Maker - The official Game Maker website where you can try the free or buy the professional version of Game Maker. They're also doing some new Game Maker Suite thing, but it's a bit confusing to me. You can check it out from the main page if you'd like. Game Maker is for making ANY type of game (even 3D ones). It has more freedom and flexibility than RPG Maker, but it also doesn't help you make any particular thing by giving you a preexisting engine. Making a RPG game would be much harder in this, but still very possible.

Character Generator - This can help you make sprites for your games.

Spriters Resource - For those who want to take sprites from preexisting games to use as-is or to modify them.

RPG Revolution - A great site/community for RPG Maker stuff. Has resources and people who can help you if you run into problems trying to make games or just have questions.

RPGMaker.net - Similar to the above.

VGMusic - A place to find midis of various video game songs.

GIMP - A free and powerful image editor comparable to Photoshop.