Kerbal Space Program is now an "Early Access Game"
available on Steam
Kerbal Space Program on Steam
If you haven't seen it before and don't know what it is about ... do youself a favour and D/L the Demo.
Essentially, you build rockets and space ships but underlining the giggles and explosions and the manic smiles of Jeb lies some real physics and some deep science.
You can actually accidently LEARN something if you're not carefull