Holy mother of Gandalf! Some amazing guy called Arthur Sacek has built this 3D milling machine entirely out of Lego. Be sure to watch the video till the very end, when he reveals the resulting model with a vacuum cleaner.
It may be slow and it may be using foam as its raw material, but it's not less amazing because of that. The accuracy of the subtractive milling process—the same that Apple uses for their unibody manufacturing—is really impressive. Even more so considering that the whole thing, except the metal tip, is made out of Lego bricks.
I like to say that there's no limit to what you can do with these magic bricks and, every time something like this comes out, I giggle like the Lego nerd I am. [Thanks Karl!]