DoA2 for the Xbox is a really good fighter. It is an enhanced version of the PS2 one. Mostly enhanced in pervy ways, but also a much needed change to the counter system. DoA3 and 4 kind of suck so I wouldn't really bother with them.
Culdcept Saga is an amazing mix of Magic the Gathering and Monopoly. Lost Odyssey is a very fun RPG, and the original Dead Rising is a blast. How it compares to the sequel I dunno.
Splosion Man is a kind of fun platformer, although the co-op levels don't really work that well.
Castle Crashers is an amazing beat-em up. I know it is coming to the PS3 at some point, and might already be out. If so then pick it up pronto. Oh also, the XBLA has the old school Turtles arcade game on the cheap. Much, much better than the Turtles in Time re-make if you ask me.
