IN 2004, a group of ambitious modders turned Unreal Tournament 2004 into a top-down co-op shooter. As you might expect, this mod was called Alien Swarm and was fairly popular, winning several awards, including Mod of the Year (Gamespy).
The development team then went on to announce a Source (the engine behind Valve's games) port of the mod, but then went silent. It turns out Valve hired the original team, which has since put in some hours to turn Alien Swarm into an excellent stand-alone game.
It’s here, and best of all, it's free.
Alien Swarm puts the players in the powerarmoured boots of four marines trying to make their way through an infested colony. In short, you could describe the game as being a cross between Left 4 Dead and Aliens or Starship Troopers.
While there is a single-player practice mode, the game truly shines online. The game has a fair yet challenging difficulty curve perfectly suited to four players. The game includes a single official campaign, which has a nice variety of challenges and locales. Players can unlock additional weapons and equipment through co-op play, all of which are interesting and don't unbalance the game or leave lower level players at a disadvantage. Lag isn't much of an issue as well, thankfully.
It's not perfect, though. As mentioned, there is only one official campaign at the moment, which can grow a bit tiresome after a while. Luckily, there are tools included to make your own campaigns, so we should be seeing more soon.
If you have a copy of Steam, there is no excuse not to grab Alien Swarm. If you don't have a Steam account, this game is a great reason to get one. It's free, and an absolute blast to play with your friends.