Playstation All Stars Battle Royale seems to still be pretty popular.
Portal 2 has a pretty fun, challenging co-op mode.
Not that I care for it, God of War: Ascension has a MP mode that players there find to be fun.
Assassin's Creed III, for all of its SP bugs and glitches, is an absolute blast for MP.
And as has been pointed out by others... Uncharted 3, Starhawk and Warhawk still have active communities.