Meet the Cthulhu Shield: an open source solution that lets you interact with computers using your tongue. Simply attach the device to a 5V Arduino dev board, and you are ready to send signals to it through a flexible array of 18 electrodes.
This tongue I/O kit is useful for those developing solutions for people with disabilities. As SparkFun explains:
Using an 18 electrode grid to tactilely display signals on the tongue, the electrodes on the array can be activated with patterns of electrical pulses to depolarize nerve membranes in the tongue to create different types of touch sensations. You can use these touch sensations to draw shapes or simple images on the tongue, feel different sound frequencies, receive turn by turn directions with your tongue, send keystrokes to your computer.
You can attach other sensors to create more complex projects with this.