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Alien Swarm is not exactly a secret tip. If you already have steam, you probably already know and played it. If you don't have Steam, you will have to download their client, because you won't be able to play AlienSwarm without Steam installed.
I know, installing Steam may be a downside, but if you like coop-gaming, this game is really worth some hours of your time. There aren't too many cooperative PC-games out there. This one has great graphics, great creature-design and a good gameplay. The atmosphere is perfect, the sounds are well done. And its free...
Steam released this game with an editor and I think they hoped Alien Swarm would be a community success concerning mods, like before the half-life series. I don't really know, how many mods are out there already, but surely you will find some. The game itself comes with one campaign, which can be played through in about 1,5 hours.