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.)
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.