Minetest. Minetest is an infinite-world block sandbox game and a game engine, inspired by InfiniMiner, Minecraft and the like.
It has been in development and use since October 2010. I was flying around in my U.F.O. with a load of fuel in my inventory searching the deserts for pyramids. – The interesting part is: I found the pyramid half-buried in the sand! Moving Blocks!