Worldcrafter

http://www.worldcrafter.net/

Worldcrafter is a sandbox game in the style of Minecraft.  You control an avatar who can move and jump around the world, which is completely different each time you start.  The game is designed to be open-ended, so players can decide how they want to play.  You can construct buildings or traps, grow crops, go exploring, survive a zombie onslaught or go on a quest.