ModNation ever since last year's E3 demo, but it's lived up to my hopes. It's great!
It's a fun kart racer, with balanced power-ups and smooth controls, and a few tricks that sometimes mean the race leader loses, right at the last second. Everything you expect from a kart racer game.
But the track-creation tool is the really awesome part. (Well, that and the car creation tool, or the character creation tool, actually.) Like any good level generator, your first few are awesome because you don't care, then the next few are all messed up because you have something in mind, and then the rest after that work because you find a groove and the tools don't get in the way. Then you go multiplayer and play your maps with other folks. And then folks tell you what works and doesn't work about them. And then you make more! You gotta try this.