Moonbase Alpha, a multiplayer simulation co-developed by the National Aeronautics and Space Administration (NASA) and Virtual Heroes.
While the game is not technically an MMO as of yet, it will eventually feature a massively multiplayer version, in much the same way that the forthcoming Torchlight MMO started life as a smaller dungeon-crawler. Moonbase Alpha looks to immerse players in a "futuristic 3D lunar settlement. Their mission is to restore critical systems and oxygen flow after a nearby meteor strike cripples a solar array and life support equipment. Available resources include an interactive command center, a lunar rover, mobile robotic repair units and a fully stocked equipment shed," according to the game's website.
The title currently features both single-player and two-, four-, or six-slot multiplayer gameplay. You can download it free of charge at the NASA website or on Steam. Don't forget to check out the gameplay trailer after the cut.