JesseL brought this one onto my radar, and I've been having a lot of fun with it.
The game is basically model rocket building. You can either build a rocket that'll just go up and then crash, or make something that'll be capable of landing on moons and other planets before returning.
I've managed to land on a moon a couple of times, but never had enough fuel to take back off. Still working on that one, till then there's a few stranded Kerbalnauts up there.
Given school and personal life, they might be waiting for a while. Quite a few have also died on launch pad explosions, mid-air break ups, high altitude space jumps, escaping Kerbal orbit and getting stuck orbiting the sun as a new asteroid, or capsules that I forgot to put parachutes on.
Very fun game!