Best mods for co-op gaming

Alien Swarm
Game: UT2004

This total conversion turns UT2004 into a top-down shooter where a squad makes their way through an abandoned station. It's close to Doom 3 in atmosphere, the action involves casting flashlight beams into dark corners - and twitchy shooting. It's best to keep the team together for those dramatic last stands when the overwhelming alien numbers wash over the server. The complementary weaponry, such as the heat-tracking turrets, flamethrowers and grenades, make a game built for co-op.