co-op story/campaign games

Killing Floor, Modern Warfare 3

borderlands ?

Borderlands (2) and Shank.

Borderlands, L4D2, Action-RPGs (Diablo, torchlight 2), Dungeon Siege 3.

Borderlands (play the first one first, then second).

WoW is pretty fun with friends if you aren't a "hardcore" player.

Every Co-op game

Splinter cell conviction , Double agent .. the coop is awesome

Sven Co-op is co-op goodness. All of the maps are made by the community and some have stories.

Synergy (+ HL2+Episodes)

Splinter Cell Chaos Theory + Conviction

Borderlands 1 + 2

Torchlight 2

Saints Row 3

Portal 2

etc etc etc

The Darkness 2 has a separate co-op campaign and some bonus co-op missions.

For co-op magicka is very good. story, challenges and arena support co-op.

Borderlands, Dead island

Synergy(hl2), Portal 2 and Saints Row 3

Project Eden
One of the best co-op games I've played. It actually matters what character you use unlike most co-op games these days.

Borderlands.

I can think of Trine 2, F.E.A.R. 3, Inversion.

Pong

borderlands, dead island, synergy, eye divine cybermancy, trine 2

Serious Sam. Just awesome.

GTA V, Borderlands 2, Splinter Cell Blacklist, Resident Evil 6

Fallout! I always play fallout with my imaginary friend!

Revelations 2012
/thread

Borderlands 1 and 2, Magicka, Dead Island, Dungeon Defenders, Saint's Row 2, Portal 2, Sanctum, Torchlight 2.

Co-op story is silly. Two people do not sit down to read the same book.

Borderlands 2