7. TIE Audiosurf and Beat Hazard
I know, I know, this is kind of cheating, but I honestly couldn't decide which game to put above the other. They both have a similar concept; they take your music and base the gameplay around it. However, they have significantly different mechanics. Audiosurf is like a mix of Tetris and Mario Kart, with some seizure inducing visuals thrown in for good measure, and Beat Hazard is like a much more advanced version of Spacewar!, and possibly ups the seizure visuals even more. Both play like a charm, and to play a monstrously fun and intense game based around your favourite music never gets old. The only trouble with these games is, after you've played through your entire music library, there's not much else left to do, except strive for 100% completion, or just download more music. At any rate, both Beat Hazard and Audiosurf are incredibly fun games that you should check out. Audiosurf and Beat Hazard are both available on Steam for $9.99.