Left 4 Dead 2, Killing Floor or Payday 2 would be my first choices.
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Esoterick
TacMaster
Various game modes in Garry's Mod, if you're desperate maybe the spec ops or survival mini games on Call of Duty?
b1n0gHo5t
Trouble in Terrorist town a Youtube favourite lol
Lozart
Or if you want proper games and not kiddy shooters - Ghost Recon Advanced Warfighter 2 or Future Soldier. Operation Flashpoint:Red River or OpFlash
ragon Rising all have multiple slot co-op in campaign mode
ragon Rising all have multiple slot co-op in campaign mode