Ok, so technically this is a game but it's an SF gearhead's ultimate toy.
In KSP you are responsible for the space program of the planet Kebin, and it's inhabitants the Kerbals. It's a sandbox game in which you build your own rocket designs by plugging together rocket parts to build anything from reasonably close analogues of historical rocket designs, to wild and wacky monstrosities of spacepane, shuttle, rocket and space station parts and watch them careen and gyrate madly on their way to either space or fiery destruction or both. There are tutorials and a set of pre-built rockets, probes and such to get you started as well as some 'training videos' on the site. There are also a ton of instructional and often hilarious gameplay videos on Youtube.
Kerbin is a small planet with moderate gravity, so it's actually possible to slap together a rocket design by eye and have a reasonable expectation of getting it up into space, perhaps after a few false starts and design adjustments. It's huge fun, has moderately realistic physics and plenty of moons and planets for you to try to land probes, rovers or even intrepid Kerbalnauts on. Apparently there are also 'easter eggs' hidden on some of the plants for you to find.
Highly recommended.Kerbal Space Program
Check out StarBase
, the open source science fiction campaign mapping application.