Would you like to travel to the Moon, set up a permanent base, and defend it against mutant zombies mind-controlled by the nazis who escaped Earth using UFOs back in 1945? Then, don't play—the free—NASA Moonbase Alpha.
Sadly for some, NASA Moonbase Alpha doesn't have zombies or nazis to destroy with antimatter guns. Although that's always a good way to spend an afternoon on the Moon—or Brooklyn, for that matter—NASA's simulation is quite scientific. The only purpose of the game is to set up the first lunar base, teaming up with your friends over the internet—up to six players—to set up all kinds of NASA structures, gadgets, robots, and even a fully functional rover that uses lunar physics to transport astronauts and materials.
The game features accurate simulations of both NASA technologies and lunar terrains, and offers the possibility for teams to compete against each other through the use of leader boards. NASA's Moonbase Alpha also supports voice communication, hopefully one that introduces plenty of static so it sounds like Armstrong and Aldrin talking from the Sea of Tranquility.
The game is available for PC on Steam, completely for free. That is a lot of value, since the only way you're actually going anywhere near the Moon in the next two decades is by becoming a Chinese or Indian astronaut. [Steam]