Suggest us some COOP games.

I Wanna Be The Boshy
I suggest playing the mini-game to activate the download link, it gives you a tiny glimpse of things to come.

This game is a fan-made I Wanna Be The Guy spinoff that includes multiplayer. You cannot see the exact same things as the people you play with, as their screens will be synced to them, and yours to you, but you can see where they die/respawn, where they are on the screen, and you can temporarily use each other's save points in order to help each other boost past difficult areas.

Dungeon Defenders and Magicka.

Since you already have RTS in hte list, Company of Heroes.

Otherwise E.Y.E Divine cybermancy, Dungeon Defenders, Magicka, Lara Croft and the Guardian of Light...

Lord of the Rings : War in the North:

Not the best game, but still worth it. I played it with a friend and we liked it. But you should be a fan of LOTR.

Magicka can be loads of fun in coop

Alien swarm's free on Steam. Not overly long but pretty good.
I've also recently played Supreme Commander - Forged Alliance coop skirmish against the PC. Get the Sorial AI upgrade and they're a decent challenge. Also get the Total Mayhem mod, or Experimental Wars if big units are your thing.

portal 2
magicka
serious sam 1/3
Lara croft GOL

My friends and I are all about coop play, so I can relate. Just from what we've played in recent memory that you didn't already list: Ghost Recon Advanced Warfighter 1 & 2, H.A.W.X 1 & 2, Splinter Cell Chaos Theory & Conviction--those all tend to get reinstalled from time to time for "just one more" play through ;)--Diablo II, Renegade Ops, World in Conflict, Sol Survivor, Sanctum, Age of Empires 2 (far superior to AoE3, IMO). Those should keep you busy a while.

Hmm Call of juarez the Cartel?
but it's not worth i think.
Try hammerfight it's not shooter but the gameplay unique.(for coop you need 2 mouse and a guide)
Trapped Dead

Shoot Many Robots, Diablo 2.

http://www.co-optimus.com/

splinter cell conviction, one of the most fun co-op games I've played so far but it is really short so take that as you wish

Dungeon siege 3

Portal 2?

Uncharted 3...if you have a PS3

Tom Clancy's Rainbow Six: Vegas 2, Red alert 3.

runescape?? LOLOL

PORTAL 2

Apparently A Valley Without Wind got some Multiplayer (not sure if Coop, but the game is nice)

TRine

Tom Clancy Splinter Cell Chaos Theory

Co Optimus is specialized in that, and there's also the Advanced filters when looking through the Steam catalogue - you can then select Co-op.

TIL there is co op for System Shock 2.

magicka

Also is Rage a good game for COOP? Installing it right nao' ^^