Minetest
The full source code of Minetest, and all of its artistic assets such as textures and sounds, are distributed under a variety of free licenses, making it easier to create changed versions and derivatives. The base version of Minetest provides an interface for games and mods written in Lua. Games define the basic rules of the game world, and are a core feature of Minetest. Most games feature sandbox gameplay focused on construction, mining, and creativity. Mods are used to further customize different parts of gameplay, and are an permanent part of Minetest. Mods are server-side and work out of the box when playing on Internet servers, with no manual installation needed.