These link are a must for everybody that want to get involved in GBA programming.
GBADEV - Tools, demos, documents, a great devellopement site for the GBA.
Devr's - Jeff Frohwein's Devellopement site. News, tools, FAQs, Links, a gold mine of information.
The PERN project - Tutorials on GBA programming. Clear and very well done.
BoyCott Advance - Another very good emulator with debugger.