MjrHuRTz has just released the Phalanx Campaign for Alien Swarm! Alien Swarm is a co-operative squad-based tactical shooter from Black Cat Games, built on top of UT2004. The Phalanx Campaign gives you 6 new missions, ranging from a Greenhouse facility to heavily infested living quarters.
Head on over to the Alien Swarm site for more details and screenshots!