Yeah, list any good co-op games/mods you know of, k?

Freelancer, SWAT 4.

Me and TD have been playing Freelancer our whole life.

Killing Floor.
Survival horror coop FPS.
Once a UT 2004 mod in 2005, now a stand alone retail game in 2009.

It's definitely worth getting, at $20, in my opinion.

It's still being supported and has some dlc coming, and lots of modded maps and such, so you can expect more than its original content.

Rainbow Six 3: Raven Shield and Project Reality for Battlefield 2 are what I play with my friends.

2 or 4 or both?

PS3
LBP (Best coop game evah, IMO)
Warhawk has split screen online, which is fun to mess around with.

PC
Trine

Killing Floor looks pretty good.

http://www.moddb.com/mods/killing-floor

I might as well be Doctor Obvious

Left 4 dead!!!

I was gonna say Freelancer, but as it has already been said before...

Metal Slug 1,
Metal Slug 2,
Metal Slug 3,
Uh... I guess that's it.

i remember beating all of the metal slugs with my friend once.. took us a day straight.

left 4 dead is pretty good, but there are a lot of interesting co-op mods for source that you might want to look at.

UT99 has some good MonsterHunt servers and a few coop servers that I like.

Sven Co-Op

Takes you through Half Life 2 and episodes with up to 16 players. Also accepts custom maps.


Left 4 Dead

Hurrr, obvious.


Unreal Tournament

The 2004 version has a horde mode or whatever its called.


Most Any RTS

Humans vs CPU players.

The entire Serious Sam series, which includes:
- Serious Sam: The First Encounter
- Serious Sam: The Second Encounter
- Serious Sam 2

While the last one isn't quite as good as the former two in terms of gameplay, it looks a lot prettier, and playing co-op is just as much fun as the other two. Especially if the person you're playing with is sitting right next to you, so you can scream in tandem while you gun down hordes of evil monsters. Also, the boss fights are doubly awesome when you co-op them.
About the co-op mode itself, it's basically the singleplayer campaign, but with two or more players, and perhaps some extra monsters and bonus settings.

killing floor is excellent. i purchased it about a week ago, its a very fun coop "horde mode" style survival game with plenty of different enemy types and a very fun and active community(which means tons of excellent custom maps and mods). its only 20$ and i consider it a step up from l4d.

you could also take up dungeons and dragons or warhammer (or another tabletop game) if your looking for something to do with your friends every once in a while.

warhammer will cost shitloads just saying. nothing wrong with it though.