Let's see, other than the ones already mentioned, I enjoyed...
Shoot 1UP - A fun little shump that has a cool little mechanic in being able to control all of your 1up ships... at the same time.
Epic Dungeon - A real-time action rougelike. Fast-paced and easy to pick up and play.
Wizorb - A block breaking game... but hold up! It's actually really unique and fun. Also on Steam.
Sequence - A rhythm RPG. No, seriously. Also on Steam.
Tempura of the Dead - It's... hard to explain. Head juggling... platformer... with a slick retro graphics style? That'll have to do.
Laser Cat - Exploration like VVVVVV, but not really hard and there's a cat. What's not to love?
Techno Kitten Adventure! - (The sequel, not the original) ...It has kittens. Also, the music is hardcore and the look on your friends' faces when you boot it up and play it is priceless.
Protect Me, Knight! - The ultimate party arcade style game. My highest recommendation of the list.
...I spend a little too much time looking at XBLIGames. xp