A few years ago, a surprising thing happened; gaming developers rediscovered the power of the co-op game.

It wasn't so much a case that the discovery itself was surprising - more that they'd lost track of co-op as a viable genre in the first place.

That's not to say co-op hasn't always been around. It's virtually compulsory in RTS titles, and you can always find it buried in a single-player FPS campaigns with AI filling in the gaps. But with Valve leading a new charge in the 2008 release of Left 4 Dead, co-op has staged a comeback, and in the process inspired the masses to stop shooting each other and team up instead.

Alien Swarm, from the Valve development team responsible for Left 4 Dead 2 and Portal 2, encapsulates this co-op resurgence perfectly. Based on an Unreal Tournament 2k4 mod, and remade utilising updates to the Source engine, your team of four is tasked with achieving level objectives and evacuating whilst repelling incoming waves of aliens.

This is clearly totally different to Left 4 Dead, which consists of achieving level objectives and evacuating whilst repelling incoming waves of zombies.

To be fair, the defining differences here are that Alien Swarm plays from an isometric perspective rather than as a first-person shooter, and that it's completely free.

Yep, free. To put that into perspective, it's zero dollars down now, and zero dollars per month forever. All you need to have is a copy of Steam (also free) and the ability to download a couple of gig without going over your data limit (possibly not free).

It might not be obvious as to why Valve would go to all the trouble and expense to create a game and give it away, but Valve typically have a history of profiting out of outlandish gambles so it'll come as no surprise if Alien Swarm eventually pays for itself with new Steam subscriptions.

You have four different character classes to choose from; Officer, Special Weapons, Medic, and Tech. Each class has two different individuals that can be selected, allowing for some decent experimentation as to character outfitting.

Each character can choose from an array of weapons, secondary weapons, and offhand items. Some are specific to each class, and many of the better weapons will need to be unlocked by gaining experience based on your online abilities. Some of the more powerful weapons include the flamethrower, grenade launcher and even a chainsaw, not to mention the extremely useful deployable sentry guns which improve with time as well.

Offhand items include the requisite healing kit, as well as a welder to weld or "unweld" doors, and other methods of destruction such as smart bombs and incendiary mines. In short, before you even see an alien, chances are you'll be packing more heat than a Mexican drug cartel.

Along with the experience system, there's also 64 achievements to be unlocked, some of which are clearly angled towards the more dedicated out there. Killing 100,000 Swarm, for example, will take an extremely long time indeed.

To achieve your objectives (the primary one being to stay alive) you'll need to move quickly, and keep an eye on where the aliens are coming from. This isn't always a particularly easy task - the aliens move quickly, and are fond of lurking around in the eerie shadows created by the mostly clever level design.

Most campaigns will see you moving swiftly to achieve the necessary objectives, followed by repelling aliens from an easily-defendable position. Although depending on the strength of your team and the difficulty level, just about anything could happen. Keeping pace with your progress is the excellent ambient music, which changes dramatically based on whatever predicament you find yourself in.

Teamwork is crucial to survive the seven missions currently available, and as Valve have released the SDK for Alien Swarm, that number can only go up. This game has an instant online community thanks to the zero-cost outlay, so servers are numerous, even if the Steam network occasionally struggles to support everyone.

Simply put, anyone with a capable PC should download this and try it.