My friend and I are in the middle of Gears of War and pondered again why there aren't too many games built around the COOP concept? Gears of War is great and a lot of fun to go through the story with a friend so i'm wondering what other games are out there you guys enjoy? here are my tops:
Dungeon Siege (the first)
Gears of War
World of Warcraft (really shouldn't count, i'm just talking COOP games)
Any others you can suggest?