Frets on Fire is a game of musical skill and fast fingers. The aim of the game is to play guitar with the keyboard as accurately as possible.
It is similar to guitar hero, but open-source.

Features
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Unique inverted keyboard gameplay style
Support for guitar controllers and generic joysticks
Includes a song editor for making your own tunes
Compete with others on the World Charts
Hundreds of songs composed by the community
Supports importing Guitar Hero™ I and Guitar Hero™ II songs
Multiplatform; available for Windows™, Linux®, Mac OS X™ and FreeBSD®
Open source, full Python source code available

Requirements
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128 MB of RAM
A fairly fast OpenGL graphics card with decent drivers
OS X Intel processor

N.B.
The site states that it works on intel processors only, but many have strangely found it works on Powerpc like G4's & G5's also.