Grievous
New member
I loved Resident Evil 5 in co-op, I'm looking forward to playing Resident Evil 6 at some point whenever I finally feel inclined.
Otherwise:
Borderlands 1/2
Magicka (to an extent)
Terraria (lots and lots and lots of fun, keep an eye on Starbound and Edge of Space (also available on Steam) if you like this genre.
MineCraft
Dawn of War 2 (Retribution is a must)
Civilization V
Anno 2070
Saints Row III
FEAR 3
Fable 3
Orcs Must Die! 2
It depends heavily on you and your co-op partner what you'd be able to get into. Otherwise Baldur's Gate: Enhanced Edition is also a good option, if you have the patience for it.
Husband and I tend to play co-op games to death.
CIV 5... you may start off co-op but it never ends co-op!