Audiosurf. Should have been way, way bigger. One of the most fun games I've ever played.

I don't know if it qualifies, but Valkyria Chronicles. Amazing storyline, beautiful art, and gameplay that's a lot of fun.

It's kind of funny, but if you asked me this awhile back, I would have said Borderlands. I used to preach that game to anyone who would listen. However, it looks like a LOT of people bought the game when it was cheap, as BL2 pretty much exploded. I'm glad to see it made the big time.