Kerbal Space Program is a space-flight sandbox simulator, in which you build your own spaceship from stock parts, then try and handle the reasonably realistic physics of launching, orbiting, and - at least in theory - travelling to and landing on other worlds. So it's essentially (at least to my mind) Minecraft In Space - which probably explains why I enjoy it so much.
As with many of the games I play (albeit not all), KSP has a free demo, available for download from their website. At present, this is all I have. The hundreds of parts available in the full version are trimmed down to 25, the massive solar system described on the wiki is cut back to one planet (Kerbin) with a single moon (Mün) - and it is still wildly entertaining.
Even if I'm not actually very good at it.