A far underrated engine/game in my opinion:<p>* Educational use: <a href="https://wiki.minetest.net/MinetestEDU" rel="nofollow">https://wiki.minetest.net/MinetestEDU</a><p>* Contributed mods: <a href="https://www.minetest.net/customize/" rel="nofollow">https://www.minetest.net/customize/</a><p>* Servers (all nonprofit): <a href="http://servers.minetest.net/" rel="nofollow">http://servers.minetest.net/</a>