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.
BIG mention to Sonidarc, who's very much into 8-bit retro pixel art and also runs one of the best Manic Digger servers!!!