Wearable Arduino mod for your headphones. Non-invasive procedure for headphones that don't have room inside the drivers or you don't want to hack apart your high end cans.
Adafruit just put out a tutorial for adding pattern blinking LED lights to headphones. This instructable takes it one step further.
This project is really the Adafruit Ampli-tie project morphed into a cool sound reactive LED light appliance for your headphones. It provides a visual audio meter or color organ that responds to the volume/amplitude of the audio heard in your headphones.
Daft Punk, call me.