Good games for co-op online (2 players).

Dungeon Defender's, Call Of Duty (Black Ops I, MW/COD 4), Awesomenauts, TF2, Ragnarok Online (well until they gutted the group quests)

http://www.co-optimus.com/system/4/pc.html

/thread

Never played but i think Trine looks awesome at coop.
And i recommend Portal 2 :D

the best are already said. I can add painkiller hell & damnation,resident evil 6/5.

SC blacklist's COOP is nice.

Orcs must die!2

Bientôt l'été

In the last couple of years my and my GF collected some fun co-op games:
Dead Space 3,
LotR: War in the north,
Portal 2,
Dungeon Defenders,
Trine 2,
F.E.A.R. 3,
Left 4 Dead 1/2,
Dead Rising 2/Off the record,
Borderlands 1/2,
Dungeon Siege 3,
Resident Evil 5,
Magicka,
Mass Effect 3,
Some CoD games also have fun co-op challenges,
Torchlight 2.

Resident Evil: Revelations in Raid mode would be pretty good.

killing floor is a fun game with friends, if you want a challenge just try a harder difficulty and you will die really fast if you dont work as a team