Minetest is a free open-source voxel game engine with easy modding and game creation. It is a freeware building block application that's customizable.
Here are some screenshots to illustrate the features and user interface of this application:
Sandbox game that uses Minecraft mechanics and gameplay and allows you to build anything. Screenshot of Minetest - 1504px · 1060pxBuild your own world. Screenshot of Minetest - 1504px · 1060pxUnlimited amount of building blocks. Screenshot of Minetest - 2194px · 1340pxUnique set of features. Screenshot of Minetest - 1677px · 1149pxMinetest: Relaxing to play. Screenshot of Minetest - 1677px · 1149pxStart crafting with a fellow player. Screenshot of Minetest - 1677px · 1149pxSmooth texture packs. Screenshot of Minetest - 2209px · 1337pxLimitless creations. Screenshot of Minetest - 2209px · 1337px