What are some good co-op/multiplayer games?

Portal 2?

Magicka: kind of a buggy game to be honest but so much fun playing co-op. Has story, challenge and pvp mode. It's seriously just a blast trying to get through the enemies and then blowing each other up for fun.
Castle Crashers: Side scrolling awesomeness. Enough said
Awesomenauts: MOBA game like DOTA and League of Legends except it's a side-scroller. Odd? Yes. Fun alternative to other mobas? Definitely.
Trine 2: If you like puzzle platformers then this is great for co-op. Better than playing single player and having to switch between the three characters.

Castle Crashers, Awesomenauts, Borderlands 2

I had a lot of fun with the mass effect 3 coop mode. I'd still be playing it, but Borderlands 2 took over :)

Left for Dead
Portal 2
Garry's Mod
Payday the heist
Borderlands
Dead Island
Dead Rising 2
and Farming Simulator 2013 (That's a joke)

Torchlight 2,Bunch of Heroes,Hoard,L4D2,Orcs Must Die!2,

http://www.steamgifts.com/forum/search/co-op
http://www.steamgifts.com/forum/search/coop

Borderlands, most Point and Click RPG's. Left 4 Dead, Half Life games with co op mods (Synergy, Sven's mod).
One of my personal favorites that I've really gotten into is the assault co-op mod for Far Cry.

Bloody Trapland, Payday: The Heist, Portal 2, Left 4 Dead (1 or 2), Borderlands (1 or 2), Castle Crashers, Dead Island.

Killing Floor, Payday
2 best Co-Op if i may say :3

Dungeon Defenders and Borderlands1/2

TERRARIA!!!!!!!

Arma 3 Beta, Cs:GO,CS

BL2...but it's a hit and miss kind of deal. Some people love it, some people hate it. I for one didn't like BL1 all that much but loved BL2...weird right?

Another great co-op is Saints Row 3. It's fun to mess around with friends. Once, I used the RC remote and kept pranking my friend. I called out to him that I got him a car and I parked it in front of the shop he was in. He went into the car and he was like, "This car doesn't move! WTF?!?!?!" So I started to move it and he freaked out. I laughed and blew the car up while he was in it, killing him. :D

He was pissed off at me and kept trying to run me over with a car for the next few times we played together but then he also upgraded the RC remote and tried to prank me after. :D

Borderlands 2, Trine 2, Left 4 Dead 2. The best.

Payday: The Heist, Killing Floor, Left 4 Dead 2, Dungeon Defenders, Borderlands 1 & 2.

System shock 2

No one's said Monaco? I haven't played it yet but I really want to get it, hopefully on sale, to play with my friend. It looks like heaps of fun.

Left 4 Dead 2,
Chivalry,
Portal 2,
Killing Floor,
Payday The Heist,
Borderlands 2.

Serious Sam FTW! Dungeon Defenders is a really fun game too :)