Tools for GBA ROM hacking and game development. Both general purpose and game/series specific tools are available here.
Tools, file formats and techniques for editing commercial games (GBA and otherwise) are also discussed in GBAtemp's DS ROM hacking section which will have additional information.
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.
Only thing drive me crazy is all these new horror games nowadays is its so dark, all time. I can't even play them. I understand they trying to create spooky but be creative a little, not just darkness with pop out scare scenes.