Luanti (Minetest)

The open voxel platform formerly known as Minetest, with a huge Lua modding scene.

Pro tips

1. Luanti runs many different games. Pick Minetest Game or another from the Content tab when you create a world.
2. Start in creative mode to learn the controls, then switch to survival.
3. Place torches as you tunnel and never dig straight down: caves and lava lurk below.
4. Try a small mod before installing a big pack so you can trace any problem.

Image from the project's own page, README or Wikimedia Commons, owned by its creators. Source