Frets On Fire is a free Windows/Linux/OS X game that is very similar to Guitar Heroes for the Xbox 360. The main difference being is that instead of using a guitar you hold your keyboard like you would hold a guitar.
This makes you look very stupid but is great fun to play. Below is me playing:
I couldn't get the current version to work on OS X Tiger which is why I'm playing on my PC. You can remap the controls so you don't have to hold the keyboard like that but where is the fun of pretending to be a rock star then?
The game has three modes of play, easy (as featured above), medium and Amazing (as featured below). I've yet to master easy though some songs are easier than others.
The game comes with three songs however there are hundreds more available to download online giving you hours of fun. At the end of each game it gives you a rating and tells you how accurate you have been.
Also included is a song editor that lets you edit current songs or import new songs. The requirements are basic with the game only needing 128mb of memory and an Open GL compatible graphics card. Frets on Fire can be downloaded for free off their website.