Did Steam-users check out the new Valve-game "Alien Swarm"? It was released yesterday and is actually a portation of the UT2004-mod with the same name. Up to 4 players co-op alien shooting action, with several guns and equipment to choose from. The best of all: It's completely free to download via Steam.
The game only comes with one campaign of (I think) six maps, so if you play on "Normal" difficulty, you'll be done pretty soon. Hopefully they're going to release additional maps/campaigns - and with "they" I both mean developers and players, since the whole game came with the free source code and SDK.