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Check out minetest open source too, which is similar but separate game like minecraft. You can self-host the server.

or, as the kids like to call it, the "We Have Minecraft At Home" version.

Tell them they can just call it "Luanti" now ;-) [1]

It's also becoming more and more "not just Minecraft". For instance this entry from the latest GameJam, which is a nice little shooter [2]