Atmosphir

The best summary for Atmosphir is: "You will have fun." Atmosphir is not just a game, it's one big level creator with a built-in level browser, so you can share your levels and play levels by others from around the world. The game, by Minor Studios, is currently in private beta, but will be released at the end of this year. If you can't wait you can apply for beta testing right here.

Their site is: http://atmosphir.com/
Platform: Mac OS X, Windows and Linux