I recently discovered a cool free open-source Guitar Hero clone for the PC called "Frets on Fire." You can either use the keyboard or a PS2 to USB adapter and a regular Guitar Hero guitar controller to play the game. There are THOUSANDS of songs available and since it is open-source there are many excellent modifications available.

You can download the game from here:
http://fretsonfire.sourceforge.net/

The Frets on Fire fan-based forum (where you can download a ton of songs and mods) is here:
http://www.fretsonfire.net/

Universal adapter for PS2 to USB (that works with Guitar Hero controllers):
http://www.redoctane.com/universaladapter.html

You can also buy Guitar Hero guitar controllers (with no Guitar Hero game) from the redoctane.com site listed above.