Minetest

Minetest is a near-infinite-world block sandbox game and a game engine, inspired by InfiniMiner, Minecraft and the like.
Minetest is available natively for Windows, OS X, GNU/Linux, Android, and FreeBSD. It is Free/Libre and Open Source Software, released under the LGPL 2.1 or later.
Features
Near-infinite maps
With a 620003  block playing area, there's no running out of space. Yes, the world height is also near-infinite.
Multiplayer support
Play by yourself, locally with your friends, or online with dozens of players on a server.
Multiple subgames
Want to install dozens of mods at once? You can. Want to play Hunger Games? No problem!
Modding API
Modify the game and add new content using the Lua programming language.
Texture packs
Not happy at the look of the textures? Change the textures!
Learn more at minetest.net