What is more fun than to build your own rocket, crew it with a trio of little green men, and launch it into outer space? Not many things, if you ask me. (Crewing it with a mix of little green men and little green women is one thing.)
Kerbal Space Program (KSP) is a sandbox game where you help the people of Kerbin to conquer space. On the upside, they are long on raw courage, whether this is the result of enthusiasm and pioneer spirit, or just a sign of not having any grasp on the concept of violent explosive death, is left unsaid.
As a sandbox game, Kerbal Space Program has no set goal. You build your rocket (or "hopefully flying contraption") from a set of parts, launch it wherever you want to go, and if you built it right - it'll end up where you want. At the beginning, though, your rockets end up being single-stage-to-crater suicide missiles for the little Kerbals.
The game is in continuous development. The latest update includes a full solar system with a number of planets orbiting Kerbol, the sun of Kerbin. You can go to any planet, if you can build a rocket powerful enough to get you there, or if you can be smart enough with transfer orbits to slingshot your way to your goal. Note this: spaceflight in KSP is not like in the movies, or in aracde "space flight simulators" where you simply point your ship where you want to go and throttle up. The engines in KSP behave much like real rocket engines and consume fuel at rates you would expect. Getting from point A to point B in KSP is rocket science - albeit simplified rocket science, and you get excellent help using the brilliant "maneuver node" system.
For us moon program nerds, this is of course as close to a heaven on Earth as we get in front of our computer screens.
KSP is a game for tinkerers, so despite its single-player nature, it is no surprise that it has resulted in a vibrant online community[a]. Nor is it a surprise that it comes with a plug-in architecture and a host of user-made extensions[b] (also available here[c]) to the game.