There are several hubby/wife teams wandering around the NWN server I sometimes drop into. NWN 1 and 2 are not too bad for multiplayer co-op, deffo better than BG2 which is a lot harder to play fluidly in multiplayer because it's so hard to play in realtime (And pause-spamming makes it very annoying to play!)
Mario Kart is great fun 'tho.. Wii Tennise, Bowling etc., Mario Galaxy, Super Smash Bros, although these are't really co-op are they.. hmm...
Red Alert 2 and 3 have co-op campaign modes. Enjoyed playing RA2 co-op, but haven't tried RA3 because the DRM stops it working on my system.
Freespace 2 and XvT have quasi-co-op modes. XWA doesn't, which is a damned shame.
All of the Lego StarWars games are excellent 2-player fun.
Oh, and if you don't mind ultra retro, Bubble Bobble! \o/
(You will never get the music out of your heeeaaadd....)
