I heard of these about a couple of years ago – and was simply facinated.  Here is a post I found at Dev Hardware:

Back in 2004, Bowyer set an idea in motion to create a 3D “‘printing” machine, along the lines of an industrial rapid prototyper that creates solid shapes from extruded molten plastic. Commercial versions of these machines are nothing new, and are available easily enough as long as you have $30,000 or so to spend on one.

The important thing for Bowyer was to create a version that just about anybody could afford. He figured the best way to achieve that was to design a machine that could replicate itself, and to minimize the barriers to entry by releasing all of the designs under the open source GNU General Public License.

[kaltura-widget wid="7n2tvsoxeu" width="410" height="364" addpermission="0" editpermission="1" /]

So basically this thing can print itself.  Wow.

Also – I remember them speculating this could be a new model for how structures will be created as well.  XXL printers would be constructed “around” a building site and then the printer would go to town and just build.

It’s an amazing concept and seems to be coming into reality.