We've seen some ridiculous creations in games where level editors are involved. Portal's version of this that was released yesterday is already beginning to show some interesting level designs of its own.
This Portal 2 player managed to construct a binary counter using the familiar in-game items. The DLC's item limit forced him to end the project with a binary readout, though he was hoping to take it further with a numerical display.
This might not be as monstrously complex and powerful as some more ambitious Minecraft projects, but I imagine this is just the beginning.