Originally released as a mod for Unreal Tournament 2004, Alien Swarm has been bought, refreshed and released by Valve as a free mod for their Source Engine. Being free, there's absolutely no reason to not download this game and give it a shot. According to my Steam profile, I've managed to put in 12 hours of play into Alien Swarm since it's release 10 days ago. That's a lot for someone like me who only manages an hour or two after work. I love this game and would gladly pay for it. Let me tell you why.
The long description: Alien Swarm is a four player, co-op, online game, that's played in the top-down perspective. The goal is to travel through various levels with four companions played by other players and complete various tasks to try to rescue a colony on another planet that's been over-run by alien creatures. There are four roles to choose from that can give each player a boost in various abilities. Included among the many abilities: shooting, explosions, hacking and healing. A large selection of weapons are available with each person carrying two to allow for some flexibility. Tactics can be discussed over the games integrated voice chat or drawn on the in-game level map, similar to football plans (though often with much more penis drawing).
The short description: Kill massive quantities of aliens with three buddies.
For a free game, the quality is outstanding. Alien Swarm can be compared to games that many other companies wouldn't flinch at charging $50 for. However, being free, there are some downfalls: There isn't a single player campaign or an offline campaign (there is an offline practice mode). You can only play with three other real people, no bots. There's only one official campaign available, but with the modding tools freely available I expect many community developed campaigns swiftly appearing.
Here's why I really love this game: The average level requires about 8 minutes to complete. That means, even if you only have about 10 minutes to spare, you can drop in, play a level and pop out. Unlike many other online games, Alien Swarm doesn't require time for you to "warm up" and get into the game. Most levels start slow and build up to a crescendo at the end.
Alien Swarm is the definition of a casual gamers game. It's the antithesis to StarCraft 2. If you know my opinion on StarCraft 2, you know that's a good thing. Where StarCraft2 is a frustrating grind that requires a significant time investment to get anywhere, Alien Swarm is a relaxing walk through fields of mindless slaughter. That's not to say Alien Swarm isn't intense. The finales to the levels are often finished on the brink of death, with much yelling and laughing over the voice chat.
Once the mod community release a few more campaigns, Alien Swarm will really hit it's stride. Go download it now if you haven't. There's no reason not to. It's the best game for its price, bar none.