Ms. Splosion Man, Boulder Dash XL and Warhammer 40,000: Kill Team.
The premiere game of the trio, Ms. Splosion Man, is the sequel to the wild platformer released by Twisted Pixel. The sequel expands on the original game, complete with minigames, a Super Mario Bros.-styled overworld, and even harder platforming levels - all mixed with the manic humor seen in the original 'Splosion Man.
Boulder Dash XL is yet another update on the classic arcade game from the mid-80's. It's seen every platform from the Atari home computers and Commodore 64 to coin-op to Nintendo handhelds, it only makes sense it would see new life on the Xbox 360.
Finally, Warhammer 40,000: Kill Team is a twin-stick shooter which features co-op play, and serves as an appetizer to the upcoming Warhammer 40,000: Space Marine retail game.
The latter two games mentioned are due to be released on PC and PlayStation 3 at some point as well. All of the games listed sell for a reasonable 800 Microsoft Points.