The perfect thing for people who are looking to develop Homebrew. DevKitPro is a toolset for Homebrew development on various consoles including Wii, DS, PSP and Gamecube.

You can download DevKitPro here

I recommend installing DevKitPPC, Programmer's NotePad and anything else you'd like. Microsoft Visual C++ might come in handy too. A piece of useful advice to avoid errors; make sure to install DevKitPro in a directory with no spaces

Do not:
C:/Documents and Settings/Users/Admin/DevKitPro

This could work:
C:/DevKitPro