Good Co-op game to play with a buddy?

Looking for a good game to play co-op with a buddy,preferrably RPG,other genres are fine though.

List of what not to reccommend:
Portal 2 (Already on it)
Borderlands(Played)
Magicka(Played)
Diablo 2/Titans Quest/Torchlight etc(Been there,done that)

Gears of War 2 co-op is pretty fun.

Gears of War 2 co-op is pretty fun.

Ah yes,forgot to mention,PC games.

Left 4 Dead maybe?

Yeah,good suggestion,thanks. :P

Gears of War for the PC has 2 player co-op
Icewind dale
Neverwinter Nights
Resident Evil 5
Serious Sam
S.W.A.T. 4 - sweat 4 player co-op
Titan Quest

Killing Floor or Serious Sam are pretty fun 

Was going to say Left 4 Dead, but that was already taken.

Castle Crashers

Lara Croft and The Guardian of Light. Really fun co-op.