Minetest - Minecraft Alternative Free Game Engine
Minetest is an infinite-world block sandbox game and a game engine, inspired by InfiniMiner, Minecraft and the like with a goal of taking the survival multiplayer gameplay to a slightly different direction. Minetest gives you a beautiful map generator where you can design your environment and play around in an almost infinite-world. You can build houses, dig, climb, or just about anything you want. Walk around, dig and build in an infinite voxel world, and craft stuff from raw materials to help you along the way. It offers multiplayer support for tens of players, via servers hosted by users.
Minetest is free Open Source and its available for the Mac OS X, Windows and Linux platforms. Minetest is designed to be simple, stable and portable with the support of simplistic Modding API that supports many kinds of additions and modifications to the game. Minetest is lightweight enough to run on fairly old hardware. It currently runs playably on a laptop with Intel 945GM graphics. Though, as for the CPU, dualcore is recommended.