Luanti

Luanti, formerly known as Minetest, is a free voxel game engine. It lets you build and dig in a world made of blocks, and it also acts as a platform: you can install community games and mods from inside the program and switch between them.

The default game gives you a survival sandbox with crafting, caves and day and night, and the community offers many more, from adventure and role-play to puzzle and minigame servers. Multiplayer is built in, so you can join public servers or host one. Mods are written in Lua, which makes it simple to create your own.

It is open source under the LGPL, light on resources and runs on older computers as well as on Android. There are no ads and no paid content.