Maybe I've seriously been out of the loop (WoW can do that), but I've just recently learned about Audiosurf. It's a fairly generic rhythm game. But man is it addicting.
If you are wondering, you can pick it up on Steam for only $10. Easily worth the money. It let's you take any mp3 and turn it into a level. You take control of a ship gliding along the track which is affected by the song. There is a bit more to it, but it's really best to try it  yourself. (Or at least watch a video) I find it to be a nice break from Rockband and Guitar Hero.

Some cool songs I recommend  you try out are:

M.I.A. - Paper Planes
System of a Down - Chop Suey
The Killers - Mr Brightside
Greenday - Brain Stew

My steam account is xBlitzerx in case anyone wants to add me. It does keep track of scores for songs with a similar to XBL ratings system.