I would like to direct your attention to Valve's latest FREE offering on the Steam store: Alien Swarm.  It's like someone took the multiplayer co-op from L4D, the class system and unlockables from Killing Floor and the gameplay of Shadowgrounds and smoothly blended it into one excellent package.  It really is a fantastic game, though as a free product at launch it contains one seven map campaign and nothing else.  Several difficulty modes ranging from cakewalk to donkey punch should keep your party of four busy until custom maps start rolling out.

Unlike the launch of L4D, Alien Swarm debuted with its SDK hot on its heels.  The editor package can be installed from the Steam Tools section and weighs in at a light 250MB.  Get mapping!