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.
OpenTTD?
OpenRCT2, OpenMW, DaggerfallUnity
Granted, you have to buy the game to make use of these open source engines legally. But these open source engines free you from the limitations of DRM, Windows/Wine and run better than the original engines (support modern resolutions, innumerable bug fixes, etc.)
you found one?? please share