Kerbal Space Program.
I've only got the Demo, but it's pretty fun. It was on sale yesterday or the day before, but it's still in early development. Takes some getting used to the controls and all the ins/outs. If you really struggle a guy made a couple of very nice tutorials.
Video 1 - Basic Building, Controls, and Orbit.
Video 2 - Orbit, Space Navigation, Landing and Taking off from non-Kerbal bodies in space.
Apparently there is a lot more to the game, both the current "full" (still in development) and planned.
So far I've just done the sandbox thing, popped a few rockets into the ground, blew up a few things by making stages happen in the wrong order, etc. Also launched two small rockets into orbit. The first one is now stuck in orbit because I ran out of fuel screwing around making a circular orbit and didn't have enough to drop myself back to Kerbal. Poor little green dude is going to die up there.