I remember having a blast playing SWAT 3 co-op with friends a few years ago. Although the AI is quite nice in single player it is even better if the other members of the SWAT team are humans too. Makes for great tactical possibilities. One player opens a door while the second throws a flashbang grenade into the room and then you both storm in and accidentally shoot your friends coming in through another door into the same room... :)

I think Tom Clancy's Rainbow Six support coop