"no-intro" project
They provide useful tools, dat-files for your ROM auditing needs and a vba-over file!!!
Tools & vba-over: http://www.no-intro.org/tools.htm
Dat-files: http://www.no-intro.org/
GBATEK
Specifications
Thanks to
no$gba for this great reference.
ARM7TDMI
ARM Instruction Set Quick Reference Card v2.1
They also have (free) documentation of the whole ARM7ZDMI CPU
GBA Dev'rs
A site for hobby GBA developers.
FluBBas TechDocs (GB, GBA, ...)
http://www.ndsretro.com/techdocs.html
Official GBA Programming Manual
No download available, only for Nintendo licencees.