Trainers, translations and other ROM hacks to be applied on GameBoy Advance ROM images.
Tools, file formats and techniques for editing commercial games are also discussed in GBAtemp's ROM hacking section which will have additional information for the GBA, DS and more.
Many tools are useful across systems (a hex editor works much the same regardless of the system you are working on) so do also check out the Filetrip's general programming tools section.
My dog used to chase off my neighbors cat from our yard, till one day the cat stood its ground, and paw slapped the shit out of my dog, now the cat wonders thru my yard, and my dog don't even care anymore, he's like f that cat. Lol