Right now Microsoft is showcasing some of there Xbox XNA games on there marketplace.
The ability to download and play these preview games is for a limited time. You can only download the game launcher until February 24th, and all the games will stop working on March 9th. After that, you will have to wait until the holiday release of Xbox LIVE Community Games before you can play again.
These games include: The Dishwasher, TriLiner, Rocketball, ProximityHD, Little Gamers, JellyCar, Culture