Co-op games are my favourite!
It's so much more fun to kill aliens/zombies/robots/pirates as a team rather than an individual. Especially when you end up victorious against all odds!
That's why I'm really looking forward to Gearbox's Borderlands that is due out soon (well, any year now at least). According to all known sources, it's a 4 player co-op game, FPS view, where you play as a team to complete what appears to be a huge story. There are over half a million weapon possibilities that are generated proceduraly as you play.
Sounds pretty good eh? Kind of like a futuristic Diablo or Guild wars.
Recently they changed the art direction (for the better in my opinion) which makes it look a bit more unique compared to most fps games. It does make it look like something Square-Enix would make though.
They've just released some new screenshots today and...well, they're just breathtaking.
You can check them out here: http://www.borderlandsthegame.com