Cooperative Games (online): What are the most popular?

I really enjoy cooperative games regardless of the platform and I'm happy to see that more and more of them are being released.  It seems like just a few years ago that almost no games featured cooperative play or if they did it was very minor or local-only (eg. not online).

Anyways, I'm wondering what the current most popular online cooperative games are for all platforms (PC + 360/PS3)...

The most obvious answer is probably Left 4 Dead so please don't mention that one :)

Also, I'm not really interested in team games (eg. RTS where you play on a team [unless you're both cooperatively playing one single civilization, etc]) but real cooperative games.  However, I'd consider games like Marvel Ultimate Alliance to be real cooperative games.

For example, Resistance 2 I've heard has good cooperative gameplay but I'm not sure how popular it is.

So... what are the current [i]most popular[/i] cooperative games?

Don't get Resistance 2, the Co-op is just tacked on and isn't fun.

On a side note: I don't get why some games ONLY have online co-op and not splitscreen, it makes me sad. D:

As for some recent co-op games I've played...
L4D and Killing Floor are two co-op zombie shooters...for console I can't say much..maybe Army of Two or RE5?

I know that a lot of my friends still play RE5 co-op, although I got bored of it after 2 playthroughs, a lot of people seem not to. Along with the campaign co-op, there's also Mercenaries mode, which is sort of "kill as many zombies as you can....cooperatively"

Co-op in RE5 and Gears 2  is great, RB6:Vegas1&2 are both very fun co-op games, and Halo games are all great fun played cooperatively.  The co-op collections for GRAW 2 were all pretty fun also.  And I agree, I think co-op stuff is the most fun in general, at least as far as mp goes.

I don't know how popular it is, but my god

Saints Row 2 co-op is superb.

gears 2

"Don't get Resistance 2, the Co-op is just tacked on and isn't fun.On a side note: I don't get why some games ONLY have online co-op and not splitscreen, it makes me sad. D:As for some recent co-op games I've played... L4D and Killing Floor are two co-op zombie shooters...for console I can't say much..maybe Army of Two or RE5?"