Frets on Fire

The Open Source Frets on Fire game gives you that Guitar Hero feeling on your computer without all the nasty paying money stuff first. Download the version for your platform – Mac, Linux, Windows supported – grab your keyboard like this…

…and rock. OK, so you won’t look quite as cool as an air axe ninja, but apparently you can also hook it up to a Guitar Hero controller and your TV via a Media Center.
http://fretsonfire.sourceforge.net/