A new device developed by Kei Nakatsuma at University of Tokyo Department of Information Physics and Computing lets you use the back of your hand as a touch pad. Encased in the form of a wrist-watch, the device uses infrared sensors to track the movement of your finger across the back of your hand and translate it into the same sort of signals a mouse or a laptop touch pad provides.
This may seem sort of trivial, but there are some interesting uses this functionality could provide. First of all, since you can feel the back of your hand and feel with the back of your hand, you've got built-in haptic feedback. Secondly, in this increasingly digital world, it could be very useful if everyone had their own interface by which to interact with all the digital things in their lives. No more losing the remote or touching a filthy public number pad, you just use the back of your hand.