Mount Blade Warband (theres many PvE mods like The Deluge, ZOMBIES! or Invasion). Check it out tho :
http://www.youtube.com/watch?v=aYQL4A462x4

Orion Dino Beatdown Maybe?

Spiral Knights

Lara Croft and The Guardian of Light! It's a good co-op game with lots of adventure. ^^

Get ARMA 2 and Operation Arrowhead, install the mod Day Z and have a blast

C&C 3 or RA3

I can recommend Alien Swarm. It is a free online co-op game for 4 players by Valve. Makes really fun.

Also Trine 2 is very nice and funny, cant make something wrong with that. :D

Splinter Cell Conviction is also really good and has an own co-op campaign.

Also Rainbow Six: Vegas 2 has excellent co-op gameplay, much tactics -> much fun ;)

And also Lego Star Wars The complete saga is so damn funny in co-op. I played this with a friend, was really awesome.
I think we played it a whole day long like 20 hours. ^^

Also everybody seemes to recommend Rayman Origins. I dont own it, but it look like much fun.

Then there is Shank 1+2, also very nice. Lara Croft: The guardian of light was also much fun in co-op.

I think the most other games are already named.

I wish much fun with that :)

I own Payday: The Heist also and it's just awesome, waiting for the DLC right now.

But i also can recommend Serious Sam 3 it's much fun in coop, it should get you over the time waiting :)

Borderlands GOTY you will never have enough so the dlcs will add a ton of content.
Dungeon Defenders. Titan quest Shit you have all that, it's harder than what it seem.

Torchlight 2 (not released yet)

What about Dawn of war 2 (retribution) or Space Marine, both have a multiplayer mode defending from hordes "The last stand" In fact if you only want multi
Unepic well it's not multi, but if you liked Terraria, this will like you too.

Bunch of heroes or Space Marine CooP, Diablo III, Resident Evil: Operation Raccoon City :)

try out battleforge (its free) you summon and control an army which you can costumize and its really fun with friends
also it has pvp and pve

Even though in my opinion it's shitty game, you can get diablo 3. For now it's pve only and you can earn some money at auction house as well.

Edit. Oh and you may want to try spiral knights.

Moonbase Alpha

Splinter Cell Conviction

Go for Arma 2, for the mod Day Z, it's ultimately a coop game. Great to play with a friend!

Q.U.B.E is awsooooooomeeeeeeee and Torchlight 2 and Trine 2 if you have 2 friends to play with

no matter which suggestion will win, definitely go for Dead Island if you haven't played it before and you got friends to play it with... it's surprisingly good

Saints row 2 and 3 are fun and CRAZY

Arma II especially with operation arrowhead and then Day Z is an amazing experience

Sanctum perhaps.

Half-Life Series (+Synergy for Co-Op)!

SVEN CO-OP mod for HALF LIFE 1 :)

dungeon deffender very fun for multiplayer coop

Unreal Tournament, or Alien vs. Predator =]

Singularity is pretty good, although the MP is a bit stagnated, and Bulletstorm if you want a shooter that makes you think(to get the best skillshots) and laugh(if you don't mind immaturity)

Saints Row the third, coop.