Kerbal Space Program (KSP) is a sandbox spacecraft simulation game where players build, launch, and control spacecraft and aircraft, and manage a space program as a whole. In its current form, it is a sandbox game; goals are player defined, and could include such activities as landing a Kerbonaut on the Mun and returning him safely to Kerbin, or exploring the other planets in the star system the Kerbals call home.
KSP is developed by Squad, who use a Minecraft-like business model to sell the game while it's still under development. The current version, 0.19.1 alpha, is available for purchase on their website for $23 (US), while an older version is available as a demo containing limited parts and celestial objects.
KSP also has an active modding community found on the official forums and the dedicated community parts site Kerbal Spaceport, who develop new parts and plugins. An officially hosted community wiki is extremely resourceful for new and experienced players alike.