Minetest - Minetest is a near-infinite-world block sandbox game and a game engine, inspired by InfiniMiner...
WorldEdit - Minecraft MOD - WorldEdit is an easy-to-use in-game Minecraft map editor. Through a combination of commands and "brushes," you can sculpt your world or simply perform numerous terraforming tasks.
Roblox - Reimagining the way people come together.
MCEdit - MCEdit is an open source world editor for the popular game Minecraft.
Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
Wurst - Minecraft open source hack