Alien Swarm is a free downloadable game available now through the Steam platform on PC.
In Alien Swarm, you choose one of eight marines and team up with up to three other players and exterminate an alien threat that has wiped out a colony. Although the story is tired and familiar, the story takes a back-seat to the co-op action anyway, so you should be more concerned with fulfilling your role (Officer, Special Weapons, Medic, or Tech), obliterating the alien hordes, and keeping your team alive. Played from the top-down perspective (like Alien Breed), you will spend most of your time running around 7 different levels and blasting aliens that come at you in non-stop waves until you complete the level. Although the Tech will occasionally have to hack a door and Medics spend most of their time healing, the Officer and Special Weapons do little else then shoot constantly and sometimes reload.
If the above sounds fun to you, this may be your game. If you want to do anything else in a game, then you are out of luck! Simply put, the 7 levels can be completed in an afternoon and besides persistent experience that lets you unlock more advanced weapons and gadgets, there is nothing else to do in this game, or any reason to replay it other than completing the game on a higher difficulty or earning more achievements. Although there is an "Offline Practice" mode, it is very short and does not grant you any experience for playing it, and the AI teammates are beyond stupid. So, if online co-op multiplayer is not your thing, there will be almost no reason for you to play this game. Additionally, as with any game that requires a high level of team-work, being in a team of inexperienced players is futile and frustrating.
And that is truly it. If you have any interest in an online cooperative top-down shooter, you really have nothing to lose by playing this free game. For those of you who dislike this sort of thing, you can safely skip this game altogether.