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.