MH, I'd recommend Warhawk for multiplayer; I've found that I'm playing that online more than anything else (well, I will again when the newness of LBP wears off), because the gameplay is just so balanced and the number of the things you can do with weapons and vehicles is very diverse.
Plus, the servers are always active and full of people to play against, the objectives are very free-form and don't really require you play with other people you know and can talk to, and the game w/ the Bluetooth Jabra headset can be found for $20-30 at WalMarts and Targets right now.
It's just a very solid multiplayer shooter that you can dive into if you want to kill 30 minutes, though you can lose yourself in it by playing for 6 hours, too. There are a lot of maps, and they're VERY BIG.