Touch screen using Water.

It is truly awesome, the Minamo by Taichi Inoue (Minaemo means "Surface Water" in Japanese). By putting a monitor under the tank, you will control the mouse pointer on the water surface, it is so cool.

Check it out:

LED used as touch/light sensor

LED is a light emitting diode, it is commonly used as output device. But by connecting each end with GPIO, you can use it as input device with some software tricks. This idea is cool, and I remember the first time when I got this idea was from the cool research site "Multi-Touch Sensing through LED Matrix Displays", you can find the information here:
http://cs.nyu.edu/~jhan/ledtouch/

Recently, I found someone use the same idea to implement a LED input sensing demo with ATmega8, it is very interesting.