Created by Underworld for the new VW GTI, this iOS app uses data from the car's computer such as RPM and speed. The iPhone's gyroscope, GPS and accelerometer are used to get steering and location data that are all used in the making of your driving soundtrack that is being played as you drive.
Here is a full, real-time experience:
I know that many opponauts prefer only hearing the music that comes directly from the machine they are driving but I can't help but find this interesting.
Source and more info at Volkswagen: http://www.volkswagen.co.uk/playtheroad