GameBoy Advance Development - Development site with news, source code, hardware reviews, tools and document.
GBAEmu - Emulation and development news site. Also offers emulators, tools, FAQs, and hosting service.
gbajunkie.co.uk - Gameboy Advance Development - A site dedicated to providing information on Gameboy Advance development. It offers tools, tutorials and links to all the information you need to start coding for the GBA.
