It's a real shame that more games don't incorporate a proper co-operative mode although I imagine that it must be difficult to code co-op play that follows the usual SP narrative.
I've been running LANs with a small group of friends since 1993-94 and we tend to play CO-OP stuff exclusively now, DM has totally lost its appeal.
I prefer co-op games where you can follow the same storyline as the SP game, rather than multiplayer specific maps with bots.
I can recommend the Ghost Recon series. GRAW and GRAW2 both have 4 player co-op campaign modes (as well as co-op terrorist hunt specific maps).
A perennial favourite is Rainbow Six 3:Raven Shield. Used to be a huge number of co-op servers on the net for that.
The original Unreal allowed co-op as well - dunno how that'll play on modern machines though.