"Orbital mechanics meets Lego" would be my single-phrase summary of Kerbal Space Program. It's also absurdly addictive.

The premise is simple: you are in control of the space program for the people of the planet Kerbin, cute green schmucks called Kerbals. You design and build rockets by gluing various components together, and see if it makes it off the launchpad.

It probably won't. Then after a bit more fiddling, it'll make it into the sky before it all goes horribly wrong. But you'll make progress. A step at a time your design skills will get better, and your rockets will get further, and a few hours later you'll have your first rocket in orbit and you'll feel like a god among men (or kerbals). You'll realise you forgot to figure out how to land the blasted thing, but hey, that's what the next rocket is for right?

At some point you'll finally figure out how to do an orbital rendezvous, and suddenly you're building space stations via multiple launches, docking and undocking, making reusable space planes, conducting science missions with robotic probes and landers on other planets, or trying to figure out how to "safely" get a manned (er, Kerballed) science lab down to an island on one of the moons around the Jupiter analogue. (And bring them back, right? Right???)

This game is an insane joy to play, a wonderful hands-on "try it and see" education in orbital mechanics and other aspects of physics and so far has eaten almost as much of my life as Skyrim: not bad going for a game that's still in early access and still has entire sections of gameplay yet to be implemented.