Manic Digger is an open-source implementation of block-building games as made famous by Minecraft. Manic Digger aims to be a generic platform ontop of which you can create your own game-types and scenerios. Through server-side scripting, any type of block-building game should be possible.
A little PDF format instruction manual, information taken from the wiki, and from actual in game usage.
So MD is starting to get updates again? Interesting...