If you're even slightly into racing type games, you owe it to yourself to pick up Trials Evolution from the XBLA.
The game itself is incredible - but the real treat is the Track Editor, which allows players to create their own custom tracks. There are two versions: Lite (still a very good editor), and Pro - which gives you the exact same tools the game developers used to create their tracks. Here, you can modify physics, create your own objects, and even create your own game types. People have already created FPS, flying simulators, and even Angry Birds and Space Invaders clones. The amount of freedom and flexibility allowed in the editor is simply unprecedented on the 360 - yes, it's even more flexible than Far Cry's map editor in many instances.
Honestly, it's so good that I would have paid full retail price for this game. How many games out there allow you to download user-created content that far surpasses even the professionally developed content on a daily basis.
I loved watching people crash and burn at PAX on this game, occasionally you would get someone who knew what they were doing and I have to say it was better watching people who were beasting on it. We should do a Community Game Night on this game.