Co-op, online, multiplayer games

So i'm looking for any good games that can be played in a co-operative fashion over the internet. Something like Marvel Ultimate Alliance, not specifically that genre of game but just that style of co-op play. I prefer run and gun type stuff and action rpg's but really im up for anyhting atm. Me and my buddy are having a tough time finding a new game. We're burnt out on all the popular MMO's and the marvel game while fun only took a couple days to beat and after a weak or so we lost interest. I've been looking and looking and can't seem to find anything so hopefully you guys can make some recomendations.

Thanks,
-Zero

I've played Splinter Cell: Chaos Theory's Co-Op play..and it's awesome! You really have to be able to work together in most cases. I thought it was a lot of fun. But I guess that is the case as long as you don't end up with a bonehead partner. Mine was a friend of mine, and it was really fun, except that he was minorly boneheaded and couldn't seem to defuse bombs. (You have to cut wires at the same time in 2 different places.)

Battlefield 2 (perhaps 2142, haven't played it) has good Co Op, as long as you get a good crowd of ppl on your team. If you do, it can be real fun. Commanding a squad is ALOT of fun if everyone complies. You take orders from the Commander and then accept or deny them, then give those orders to the Squad you are commanding. It's fun!

My favourite co-op game (aside from Sven Co-op, which I haven't played in years) is SWAT4 - I like it because of how tense it is and how teamwork really is essential.

Online co-op eh?
Theres a few of them, just depends on what you like

Diablo 2
Never Winter Nights
Counter-strike (played with a mate or 2 this is damn fun when you work together)
Ghost Recon
Splinter Cell:Chaos Theory (But I beleive this also installs starforce :S)
Guild Wars

There are alot of great games out there

Yes, SS:CT Does have Starforce (So does the new one I believe, horrible IMO, but the games rock)

If you have a xbox 360 two superb co op games came out with in the last month.

Gears of War - You can play the entire campaign with a friend.
Rainbow Six - Same as above. However you miss out on the campaign cut scenes. You can play this with up to four friends online.

My friend and I definitely enjoyed Gears of War. Awesome campaign.

Second Gears of war, that game is freakin' amazing

Thanks for the heads up guys, I've got the new splinter cell but can't get the game to render properly on the 8800 GTX so i'm waiting for a patch or updated driver. I've also been hinting to the girlfriend to get me a 360 and GoW for Christmas *crosses fingers* Unfortunately my friend doesn't have a 360, may have to start dropping hints to his girl too...lol. I guess what I'm really looking for is something like the Halo series co-op but need to be able to do it over the internet. Run and gun fun or action rpg type stuff. I have counterstrike which came with my HL2 but haven't ever played it. I guess I'll check that out and if you guys know of anything like the halo campaign co-op let me know.
Thanks again,
-Zero

Run and Gun = Serious Sam First Encounter and Serious Sam Second Encounter

(Note: I do not mean SS2)