This topic has been briefly touched in the introduction thread, with Simon commenting:
SimoRoth wrote: I can't buy this game. If I did Maia would slip back about 14 months in development!
UPDATE: KSP is now availeble on Steam (here) (Windows, OSX or Linux!)
The demo has also been updated and is now on near-par with the full game.
It's about Rockets! Design/build them, and attempt to get them into orbit! Or land on one of the 4 other planets, or one of the many moons..
Or build your own Space Station, or a base! Anything!
The game uses an accurate (though slightly simplyfied) representation of a solar system in which you are free to launch rockets of any design.
Kerbals are an Alien race that recently discovered their place in a larger solar system, they are now highly motivated to conquer the Unknown. You are the director, designer, and pilot for their Aero-space program.
The game is still in early development (for example: there is no atmospheric-drag->heat system yet, you can plunge a bus-sized module through any atmosphere without it burning up).
There is no story line/career mode yet, for now its a huge sandbox rocket sim with infinite funds/parts.
The modding possibilities are huge, you can create custom parts, gauges, even entire game mechanics.
The original game does not have any life-support functionality, so someone created a life-support plugin. Plugins can all tie into each other easily so half a dozen others have created compatible tanks/solar panels to store/refresh the life-support system.
There is a large active community that shares designs, accomplisments, challenges etc..
[s]There is a free demo (v 0.13), it does not have a persistant game world yet (so you can only have 1 active mission at the time), and there is only 1 moon.
The (as of writing) most recent (paid) version (0.17) has EVA's, 4 more planets (with moons), more (and better balanced) parts, and loads of other improvements/additions.[/s]
edit: Demo is now version 18.3, full version 19.1
PS Sorry if the above sounds like an ad, but this is currently my #1 favorite game.
edit & PPS:
I'm toying with a mapping plugin, and I found this monument:
PPPS Can someone please tell me how to hide/spoiler things on this forum?