I rarely play single player actually. Although I don't know if you count taking turns to play as multiplayer.

Subtracting those games, these are the games I've played co-op or versus on lately:

Little Big Planet
Tekken 6
New Super Mario Bros
Final Fantasy 9
Blue Toad Murder Files
Obscure
Any Lego game
Boom Blox
Resident Evil 5

various old snes/nes games I have emulated on my wii, such as the Mickey Mouse series, Gooftroop, Kirby 3, Donkey Kong, Bubble Bobble etc