Build your own rocket and launch it into space!
Kerbal Space Program is a space flight simulator that gives you the task of building a rocket capable of taking a crew to the moon. It has very realistic physics and is surprisingly difficult. However, this is softened by being set on the fictional planet of Kerbin, populated by a cute intelligent alien race called the Kerbals. It has an extensive career mode, and a sandbox mode that gives you total freedom to experiment.