Minecraft - A block-building game that allows you to create and explore entire worlds from scratch.
Minetest - Minetest is a near-infinite-world block sandbox game and a game engine, inspired by InfiniMiner...
Roblox - Reimagining the way people come together.
Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
Terasology - Terasology (started under the name Blockmania) is an open source project started by Benjamin...
Voxelands - The Fun-Focused Free Software Voxel World Game.