Amaranthian

You should all get Audiosurf! It's just $2.49, for a limited time only!
You can get a free demo if you search for "Audiosurf" in Steam (Steam is free!).

The premise for those who don't know what it is: You take some music from your computer, and the game interprets it as a roller coast type track with notes all over it. You have to hit those notes and make clusters of colors that give you points! Here's a video: http://www.youtube.com/watch?v=KdItTu0MjXc
Kind of low quality, but I think it gives the gist. Also he's using a mono car which, in addition to carrying a herpes virus, has to dodge gray blocks and collect all the colored blocks (Though they change color depending on the volume of the track, they're considered to be the same color).

It's a combination of trippy visualization of music, roller coasting, and fast-paced reflexive action/tactics. It's super duper fun, and TOTALLY worth the 2 and a half bucks.