Kerbal Space Program is a simple, free strategy game in which you're in charge of an alien space program. Don't worry, it's nothing too complex. You just build rockets, and if they explode, you build a better one.
The core of the game is the space craft creation suite, which is essentially Spore's Creature Creator, only with rockets. The idea is you build a rocket, then put three aliens in it and shoot it off into space. If it crashes, and they all die, try, try again! If it makes it into space, then you can attempt to actually get them home again.
Sounds simple, but there's surprising depth to the game's creation aspect, and given the fact you need to manually control the rocket, there's some deft handling required of the launch section as well.
While the game is free for now, once the developers have it refined and polished enough for a proper release, they'll be charging people for it. Though, the free version you download now will always remain free.
Kerbal Space Program is Windows-only, and you can download it in the link below.
Download Here [Official Site]