Kameo is a really great platformer, classic Rare stuff. The game itself isn't very long, but there's a lot to do to unlock all the achievments, and it's really quite hard. They're almost constantly adding in new downloadable content for it too. The Joanna Dark costume is really cool.

A lot of people don't seem to like it, but there's a demo up on the market place so you can at least try.
Condemned is a good horror game, really quite scary and suprising at times. Project Gotham 3, Dead or Alive 4, Call of Duty, etc. Chromehounds is out pretty soon too, a mech based squad game that's looking really good. Really, the best thing to do is to go onto the market place and download some demos.