Scientists have invented a way to form 3D metallic objects at nanoscale. While it's not 3D printing, per se, it is quite interesting.
The approach was to mimic traditional methods of forming metal: bending, shearing, etc. They've found a way to deform portions of a microscopic metal object in a controlled manner. Using this approach (which you can observe happening in the video above) metal strips accurately bend according to plan.
The resulting metal objects not only retain their shape but are also sufficiently robust for use in micro machines. By combining this technique with nanoscale 3D printing, we'd soon have the ability to make many kinds of invisible machines.