I wanted to give a shoutout to the TrueCraft project lead by SirCmpwn. It is a clean-room implementation of Minecraft beta 1.7.3 (~2011 September) - including the server and client (and it’s compatible with Mojang’s server and client). It’s been and being developed from scratch without looking at Mojang’s original code, so it’s certainly an impressive feat of engineering! It’s also another completely FOSS project (like Gamocosm), largely motivated by the developer’s own enjoyment of the game. TrueCraft aims to bring back the original spirit of the game: a simple sandbox where you can explore, fight, and build with your friends.
You can create a TrueCraft server on Gamocosm - just select it in the drop down menu when creating a server (of course, you can also grab it from their website and set it up yourself however you like). Happy Minecrafting!