Minetest

Minetest is an 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.

Minetest is designed to be 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.
Features

- Walk around, dig and build in an infinite voxel world, and craft stuff from raw materials to help you along the way.
- Simplistic Modding API that supports many kinds of additions and modifications to the game.
- Multiplayer support for tens of players, via servers hosted by users.
- Voxel based lighting with gameplay consequences (light caves and buildings with torches)
- Almost infinite world and a beautiful map generator.
- Runs natively on Windows, Linux, OS X and FreeBSD.
- Supports multiple languages, translated by the community.