Link fixed. Weird ...
It's a zombie detector.
A headphone amplifier takes a source from one end, amplifies it, and spits it out the other but in the process you pick up a warm tube sound. A decent tube powered headphone amp can run 500 to a few grand. With the kit I got to build it myself and I can always try different tubes to get a different sound. I tell ya, this thing was fun as hell to put together.
This is the site I ordered the amp kit from: http://www.bottlehead.com