Anaid Gomez, a NYU student has created probably one of the coolest peripherals for your iPod, the Kokoro Heart Sensor.
The Kokoro Heart Sensor features a Polar heart rate sensor inside, what it does is sense you heart rate and then plays different types of music depending on how fast your heart is beating.
If you are excited, the Kokoro will play fast beats, if you are relaxed it will play slow beats, check out the video if it in action below.
This is pretty cool, it certainly looks like it would be a fun accessory to use.
Kokoro Heart Sensor via Engadget, Dvice