Minetest Game by Minetest

A game which is bundled by default with Minetest, and aims to be lightweight, moddable, and fairly playable without mods.

The gameplay is mostly sandbox-style. There are no monsters or animals. You start in a blocky world somewhere between the coldest glacier and the hottest desert and just do whatever: Craft a pickaxe and go mining for ores, gather blocks and resources, craft new blocks, tools and items, go farming, use your new blocks build an awesome house, and so on. Read the Minetest Game wiki page for a full explanation.