Frets on Fire is a free open source Guitar Hero esque pc game. Turn you keyboard upside down in this fully customizable guitar emulation. Change everything from skin mods, to programming the songs yourself. Joystick support allows you to program not only your keys of choice, but also joystick and guitar controllers. Links on how to build your own guitar below. Frets on Fire is available for Windows, Linux and experimentally on OS X.

Links:
Frets on Fire Official Page
Frets on Fire Wiki
Alternative song creation software:
Freetar and Feedback
Instructables on building your own guitar controller:
Easy Guitar Controller
Make a Guitar Kit
How To Make a Custom Controller
Customizing your Keyboard

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I freakin love frets on fire! and much easier to install than the other guitar hero clone i was messing with.