I really like CoD 4's multiplayer nowadays. You can go in to a match, play with the same people for hours while not having to be on the same team every match, then disband and search for another group. Very accessible, and more respectable now that all the "must have, must play" kids have run off to CoD:WaW.
Burnout Paradise is also pretty fun in a full 8-player freeburn. Just drive around, crashing into each other and challenging each other to races or Road Rules.