I'm excited about Space Engineers. It sticks you in space, able to build space stations, large and small ships, and fool around with it. Definitely a sandbox game, it revolves around physics like thrusters and gravity, and insane collision mechanics.
It is definitely in Alpha. The build is very stable, but not very robust. I've really enjoyed playing it, and can't wait for the implementation of further features - especially multiplayer. Check out the trailer: