What is the most interesting thing you have seen done with a 3D printer or 3D printing technology?

The most interesting things I've seen are the printing batteries to fit confined areas and prosthetic limbs cheaply to help children and adults in places where the just isn't the money to supply
more expensive alternatives while still giving them additional mobility and dexterity.

Technologies like this are proper uses of the technology and potentially can create devices and parts that are difficult , expensive or almost impossible to build by conventional means.
It's pointless printing items that can be produced faster, cheaper or easier by conventional means. Also producing items on sire is a much better way of utliising the raw material.
 If you could build your dream 3D printer, what would it include and what would it do? Let your imagination run wild!

It would be a combined 3d printer that can recycle and produce it's own filament if it was FDm combined with a laser cutter, CNC mill which would allow an item to be produced in the most appropriate and economical manner

The most useful addition could be a 3D scanner that could take incomplete or broken parts and like predictive text on a mobile phone suggest what the complete item was or compare parts in a massive database of parts to allow the printing of replacement parts with teh need to create a new 3D model of the part.

Also multi material to allow the device to choose the most appropriate material for the job.