By Frets on Fire is a freebie PC game very much in the same vein as the Guitar Hero series. The game comes with 3 or 4 songs included and there is a vibrant community out there. It even has a converter with it to allow you to convert Guitar Hero songs to work with it. It's extremely easy to find other songs from a variety of places on the net. Frets on Fire works on Windows, Linux and Mac OS X.
