I haven't bought this yet (I think it's $23) as it looks quite fun and I suspect it could turn into a real time-sucker!
From the website here
Introducing Kerbal Space Program!
KSP is a game where the players create and manage their own space program. Build spacecraft, fly them, and try to help the Kerbals to fulfill their ultimate mission of conquering space.
The game is currently under heavy development. This means the game will be improved on a regular basis, so be sure to check back for new updates. Right now, KSP is in Sandbox Complete state, but we want you to try it out and have fun with it. The first versions are free to download and play, and will remain so forever.
The KSP Team is strongly commited to the project, and we are always listening for feedback from the players.