The other night a friend and I stayed up 'till 5-6 ish playing Team Deatchmatch on Resistance 2 :O
The 60 player servers are insane! and there are tons of them filled to the brim! The maps are wide enough, and vary enough so you're not always swapping bullets (a little breathing time) and the sniping is so damn fun
Managed to rank first on my team once, nearly got 1/10th of the kills (team kill count was 1000 , it was a huge battle!)
Aside from the deathmatch, the coop is awesome, class based so you're looking at limited weapons, but the classes are balanced and helpful (tank type, healer, ammo giver) the healer (my choice) charges his own health and healing ability by drawing it from the enemy via a gun not unlike ut's pulse rifle alt shot. Didn't play the other classes, but on top of it all, you have leveling up and stuff, its cool!
So yeah, that was the best multilayer experience I've had in a LONG time
Y'all should try it if you have the chance