ClassiCube

ClassiCube is a clone of Minecraft Classic, it adds numerous features to the original Minecraft Classic formula, such as flying, speed-moving, noclipping, more blocks, custom blocks, and better multiplayer.

ClassiCube is free software and the source code is open, meaning anyone can download the source code and create and use their own custom version of the client, various people have made modified clients and client-mods using this source code.