Wiitar
Wiitar connects your Nintendo Wii Guitar Hero guitar controller to your Mac. Play Frets on Fire, use it as a GarageBand instrument, or just have fun.
Learn more at the Wiitar homepage.
Get Wiitar
Free. Download from Sourceforge. Requires Mac OS X 10.4 or 10.5 (Universal binary).
This project is open source. Source code is also available at Sourceforge.