Interesting little game about building your own spacecraft and then flying it into space.
It's still in alpha stages but you can pre-order it and get each new version for free ala Minecraft.
It also has a growing mod community, so it's a win win.
There's a also a free demo which is an earlier version (but it has everything you need to get into orbit and land on the moon and stuff).
Been messing around with it for hours. After half a dozen failed attempts at moon landing, I made a three stage thruster system and inadvertently managed to launch myself into an orbit around the sun. Never made it to the moon though.
Website for mods