A flurry of Co-op games

[Mehen]

Aliens: Colonial Marines (Q2 2009)
PC/PS3/X360

http://en.wikipedia.org/wiki/Aliens_(Gearbox_Software)

4 Player Co-op, each Marine has a different primary weapon.

Borderlands
PC/PS3/X360

http://en.wikipedia.org/wiki/Borderlands_(video_game)

"Borderlands an upcoming science fiction first-person shooter with RPG elements that is currently in development by Gearbox Software for Microsoft Windows, PlayStation 3 and Xbox 360." Out now! Excellent game, lots of fun, good replayability.

Call of Duty (5): World at War
PC/PS3/X360

http://en.wikipedia.org/wiki/Call_of_Duty:_World_at_War

4 Player Co-op, great game, challenges are fun, fairly short.

Call of Duty (6): Modern Warfare 2
PC/PS3/X360

http://en.wikipedia.org/wiki/Modern_Warfare_2

2-man Co-op in "special ops" mode. Not the full campaign, but supposed to be similar in length.

Castle Crashers
X360 (Xbox Live Arcade)

http://en.wikipedia.org/wiki/Castle_Crashers

4 player co-op, side-scrolling, beat-em-up with RPG elements.

Gears of War 1 and 2
PC/X360, Gears 2 X360 only for now

http://en.wikipedia.org/wiki/Gears_of_war

I'm not sure when Gears 2 for PC will be coming out, but Gears 1 is solid for the PC and features some extra content over the 360 version.

Left 4 Dead
PC/X360

http://en.wikipedia.org/wiki/Left_4_dead

4 Player Co-op, lots of awesome zombie killing!

Left 4 Dead 2
PC/X360

http://en.wikipedia.org/wiki/Left_4_Dead_2

Save money by getting the 4-pack with some friends!

Painkiller: Resurrection
PC

http://pc.ign.com/articles/971/971617p1.html

New Painkiller game with an updated engine. Features 4-man Co-op! Available October 27th.
I TRIED THE DEMO - IT SUCKS. Not the same developer as the original. Feels unpolished. Terrible voice acting. Supposed to be new engine but it looks the same or worse.

Serious Sam HD
PC/X360 (Xbox Live Arcade)

http://xboxlive.ign.com/articles/101/1011871p1.html

The original Serious Sam on a new engine. Includes 4-man Co-op. Out now!

Unreal Tournament 3
PC/PS3/X360

http://en.wikipedia.org/wiki/Unreal_tournament_3

While not specifically known as a single player game, you can go through the campaign with up to 4-man co-op. Fairly lengthy, although the maps/missions are sometimes repeated. I played through it with a couple friends - awesome game. Even better than UT and UT2k3/2k4.

[Mehen]

Added Painkiller: Resurrection and Left 4 Dead 2.

[Mehen]

Added Borderlands.

[Mehen]

Those are all PC. Some of them are console as well. I'll edit the list shortly.
*edit: added supported consoles. Also added Serious Sam HD and CoD MW2. Also now in alphabetical order!

[Mehen]

Added Castle Crashers! Looks like a fun game. I really gotta get myself a 360. The new slim PS3 is tempting though too, especially because I need a bluray player.

[Mehen]

Just beat Borderlands (1st and 2nd playthough).

My biggest complaint is that it wasn't hard enough or long enough (that's what she said).

Going to have a LAN in a couple weeks and playthough L4D2 and CoD MW2.