Minetest is a free and open source 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.
It is Free and Open Source Software, released under the LGPL. This way it is more readily available to Linux users, and random people (you!) can fix bugs and experiment effectively.
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Started during late 2011, Minetest 0.4 is an ongoing (as of 2012-03) effort to bring Minetest to the next level. It transforms the game into an engine, capable of supporting much more distributed work effort, many different game projects and enabling users to customize their copies.
This is being done because Minetest 0.3 is largely a monolithic system - it cannot be modified easily, users cannot plug in their own functionality, people can't reasonably build their own projects on it; it just plain doesn't scale like we would want to. Everybody deserves better!
http://minetest.net/
Starting from scratch.