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.
To help show people just what to expect, here's a quick tour around some of the things and places found currently on the 'exe creative' public server.
cool do you have to make the whole place or map yourself
the 'tower' and structures, the hollowed out mines in the ground are all created by the users, essentially the game generated a landscape and then people went nuts.