Now we just need to 3D print this 3D printer...
When I first saw this video of a 3D printer within Minecraft, I was blown away -- but it was getting late, and I learned long ago not to publish at night. It's since come to my attention again, and I'm taking that as sign enough. The video, created by YouTube user ItsJustJumby, was made possible by an astounding 20,000+ command blocks alone, and that's just one of the component types.
How one goes about thinking up such a project, much less putting it all together and sticking with it to the bitter end is beyond me. What dedication.
To download the world and try the printer for yourself, head over here.