Minetest is an infinite-world block sandbox game and a game engine, inspired by InfiniMiner and Minecraft. It has been in development and use since October 2010.
It is Free and Open Source Software, released under the LGPL, available for Windows and Linux.
Minetest is technically simple, stable and portable. It 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.
Official Website : http://minetest.net/
Status : Released
Platform : Windows, Linux, Mac
Multiplayer : Yes
Price : Free