WHAT IS ON THIS HOTSHOE?

Http://www.uuelf.com/Hongdak-MC-30-Remote-Switch-Cable-Style-Shutter-Release-Control-for-Nikon-D3-D700-D300-D2H-D200-D1H-D1X.html?currency=GBP

In the picture there's a camera with a small device wired to the camera… What is this? What does it do?

It looks like part of this remote kit:

Ikk the Nikon ML-3 Modulite Remote Control Set

Since this is a generic version, you will have to ask Hongdak

It's a wireless RF shutter release trigger receiver to suit Nikon's pro/prosumer cameras… The unit is just mounted in the hotshoe but the actual connection is into the multipin connector on the body.

EDIT: There's always one numbskull that hasn't a clue. I actually use one of these…

Receiver unit for a radio controlled cable release, the cable then plugs into the camera.