Nice COOP FPS?

Its a question/request too off-topic to post anywhere else, but anyone know a nice coop fps 3-4 friends can play together?

We've enjoyed together in the past:
Ghost Recon series
Rainbow six series
Borderlands
Modern Warfare: World at War
Halo
Sven Co-op for HL
L4D & L4D2

Suggested in this topic:
Serious Sam (Twice)
CoD Black Ops Zombie mode
Gears of War

Any and all suggestions and comments are welcome!

Eh black-ops zombie ?

Nothing quite brings people together, and also find your friends are assholes that you'll never trust in a real life survival situation, quite like the Left 4 Dead games.

It's a game that really has to be played with a bunch of friends. Yeah, it's perfectly playable on the normal difficulty levels with bots, but it's a whole other experience with a group of friends and even strangers.

Some old-fag recommendation:
Serious Sam(or you can try SS HD if you like)

BTW, there are a bunch of coop mods for Half-Life and Half-Life 2, like Sven Coop(my personal favorite) or Synergy and Obsidian Conflict, you should try those.

I'm just throwing this out there, don't hate:

Halo (Reach)

Dang, I've actually done everything listed here besides Serious Sam and Black Ops zombie but forgot to add them to the list. Halo, Sven, and L4D series were all fun for a while. I'll update the op.

I always find that Gears of War 2 is always fun when you system-link 2 consoles together and have a 2v2 match.

I like a lot call of duty series!

Serious Sam. You and your buddies against the HORDES OF ENEMIES RUNNING AT YA.

Chaingun, chainsaw, grenade launcher, rocket launcher, sniper rifle... it's just an assload of fun to play with your 'mates.