The game supports four player co-op play along with 64 new Steam achievements, Steam Cloud support, and an updated Source SDK for mod-makers to dig in to. Though the Source engine has been ported to the Mac, Alien Swarm is currently only available for the PC. It is possible that it will be released for the Mac at a later date.

As this is a free game from Valve, there will likely be a lot of discussion about the title. Let's get this conversation started in the comments system.