Trans-lingual Electric Nerve Stimulator
A.K.A. a tongue-zapping retainer. This project is highly nascent, but the idea is to build a dental retainer, on the bottom of which is an electrode array. If one holds one's tongue to the roof of the mouth, it will press against this electrode array and one can feel the patterns of electrical stimulation.
 Physical Design
The armature will be made with a non-toxic molding plastic fitted to a user's mouth. This same mold needs to fit over a lead-free printed circuit board. On the top of the board, completely enclosed and sealed from the mouth, are a number of wafer-thin shift registers. On the bottom of the printed circuit board, exposed to the mouth, is an array of electrodes. The power supply and logic control is external to the mouth, and connected to this retainer by the thinnest possible wire, which would extrude from the corner of the mouth.
 Component Sourcing
 Display Uses
My (Skory's) biggest interest in this device is as a way to display speech signals. I think this could be an ideal display for this because we could try to map a spectrograph of a voice to the electrode array in a way that corresponds to places and manners of articulation in speech production.
Of course, once the display is built the controller can have a flexible interface allowing any sort of signal pattern to be mapped onto it, perhaps over XBee or Bluetooth from a computer.
 Further reading
Here is a research group at the University of Wisconsin studying the display of visual information on the tongue.