Dev Kit Pro
Quick presentation of DevKitPro.
A set of toolchains of choice for homebrew game development, currently available for GameBoy Advance, GP32, Playstation Portable and GameCube. More platforms will be added as time permits. For DS, GBA and GP32 you require devkitARM.
More info at
http://www.devkitpro.org/.
You can also download the installer
here, or the files one by one
here.