Cube 2: Sauerbraten is fun and it uses very little resources. The game and engine are open source. Other than that I can’t really think of anything other than

Cube 2: Sauerbraten is fun and it uses very little resources. The game and engine are open source. Other than that I can’t really think of anything other than good clones like OpenTTD or crappy clones like minetest. Maybe Dwarf Fortress in the future if tarn or his brother open source it.