Grab Monaco and Spelunky if you can. Heard they are fantastic multipayer party games! Need to pick them up myself. I can also vouch for how great Shadow Complex was. And you can't go wrong with Minecraft (if you don't have it on PC).

Don't forget the XBLIG section - there some fantastic indie games in there, like qrth-phyl (a really crazy snake type game), Vidiot Game, Evil Quest, Cthulu Saves the World, Breath of Death 7, Penny Arcade 3, Escape Goat, the Arkedo series (really fun old school arcadey games), Shoot 1up, Flotilla, The TEMPURA of the DEAD, Leave Home, Tobe's Vertical Adventure etc...