GameBoyLinks

Game Boy Advance

http://www.gbadev.org/
Gameboy Advance Development

http://www.devrs.com/
Jeff Frohwein's Software/Hardware Dev'rs

http://www.ziegler.desaign.de/
Reiner Ziegler GBA Development

http://darkfader.net/gba/
Darkfader.net - Gameboy Advance

CowBite Virtual Hardware Specifications
Inofficial documentation for the CowBite GBA emulator by Tom Happ

GBATEK
Gameboy Advance Technical Info

Programming The Nintendo Game Boy Advance
This book was not released in book stores due to legal problems with Nintendo. As a result, the e-book edition is available for free as a downloadable PDF e-book.

http://gnufoo.org/
A GCC based SDK for the Gameboy Advance, running on Mac OS X.

Loirak
A programming/hobby development site. Contains good tutorials.

GBAGuy's Lair GBA ASM Tutorials. What is IE6?

SGADE
The Socrates Gameboy Advance Development Engine.

The Pern Project
Programming tutorials, demos, and source code for the Nintendo Gameboy Advanced.

Realtime 3D on the GBA

Lysek

devkit.tk
Toolchains of choice for homebrew game development.

Nintendo DS

darkfader.net