OK, this is potentially the coolest thing ever. The Interactive Electronic Tattoo over at Neatorama is something that's inserted under the skin and uses glucose to electricity.
The device is bluetooth and allows touch control through the skin. Imagine being able to walk up to a computer and getting a terminal login that you can type commands into, or use an iPhone like touch-screen. Or have it talk to your car, cell phone, or whatever else you can think of. Also as it's computerized you would be able to update it with new functionality....
Some potentially awesome uses for this, you just have to be OK with a permanent non-natural, electricity powered addition under your skin :)