Kerbal Space Program
From Kerbal Space Program Wiki
KSP is a game where the player creates and manages his own space program. Build spacecraft, fly them, and try to help the Kerbals to fulfill their ultimate mission of conquering space.
The game is currently under heavy development. This means the game will be improved on a regular basis, so be sure to check back for new updates. Right now, KSP is in Alpha state, but we want you to try it out and have fun with it. The first versions are free to download and play, and will remain so forever.