Minecraft can work perfectly well without a central server, provided you have downloaded the game. They do have an authentication scheme to tell the server you

Minecraft can work perfectly well without a central server, provided you have downloaded the game. They do have an authentication scheme to tell the server you own a legit copy of the game, but if that were to go down, the game would still be playable. Server software includes a "online mode" property that can easily be set to false to accept all clients regardless of central auth status. This of course opens a new can of worms but there are solutions that already exist.