Cloudberry Kingdom (also on every other console, I believe), a randomly generated endless platformer, is sooo fun. I highly recommend it for $10. Well worth it. Try the demo. Be sure to try it coop (local-only) if possible - it is so much fun coop, though it's fun solo too. The full version adds unlockable characters that change the game (the game adapts to the character, so if you pick Jetpack, you'll get levels that require the use of the Jetpack for example) and a variety of game modes (for example there's a timed mode where a clock counts down and you add time by collecting coins), as well as a story mode.
Got this last night. Awesome game so far. Just playing for 15 minutes had me laughing out loud. Especially if you grew up in the 80s, it's a must have game. And it's currently on sale for $7.49 on XBLA!
I didn't grow up in the 80's but am a huge fan of sci fi, so it still clicks with me. Real awesome game for $15, a steal at $7.49.
Guys try "Astralis" on the indie games menu. Its an indie open world shooter featurin you as a commisar fighting flying alien terrors. http://marketplace.xbox.com/en-US/Product/Astralis/66acd000-77fe-1000-9115-d80258550dbc I bought it a few days ago and it is fun. Light on story tough. Its also a buck.