This news really impressed me!
At the University of Washington researchers have found a way to exploit the loudspeaker and the microphone in a smart phone to create a sonar system that can track nearby objects.
An app generates ultrasounds that get reflected by surfaces and these reflections are captured by the microphone, exactly what a sonar system does. The app analyses the reflections and can resolve the spatial distance of the reflecting surface.
With this system you can use your finger to draw on a surface, like your arm (watch the video clip), or a pen on a sheet of paper and you'll see your movements copied on the smart phone screen. Once you have that you have basically a tracking device without the need for a tracking device!
What really amazes me is the ingenuity of researchers that can exploit what is already available to create something completely new! It really shows the power of the technology we have in our hands and its potential that is just waiting for some bright mind to exploit it.