http://kerbalspaceprogram.com/about.php'cause, who doesn't like building spaceships, right?
The game is currently in Alpha/Beta and is therefore free
Basically, you have a lot of parts with which to build your spaceship (you can download or even make more parts yourself
)
When said spaceship is complete, you can then launch you spaceship into....err...space, and even attempt to land on the moon.
