If you haven't heard of this before, shame on you.
It's a game being made by a single person, Markus Persson(AKA Notch). It has a creative and a survival mode. The creative is simply sandbox freedom, where you can do whatever you want. In survival, you have to brave the elements.
Creative currently has a multiplayer version, and survival has one on the way.
http://minecraft.net/
A few of your staff from here play it with me, on the WoM(
http://worldofminecraft.com/) servers.