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.
Props to Flauchig for his 'floating castle' build which was most impressive until it got griefed out of existance.