Heart Beats continually modifies the tempo of your music to match your heart rate. Hopefully this won't result in a positive feedback system that gives the user a heart attack.
It uses a heart rate monitor that wirelessly communicates with a watch, and there is a webcam duct-taped to the watch with quick'n'dirty OCR to read the heart rate in Max/MSP. The heart rate is divided by the original tempo of the song, which determines the speed at which the song should play.
Bonus hack: I saw the HarmonicScore team's work and noticed that their image output (a noteroll generated via MIDI data, with harmonics added in) looked very similar to a spectrogram, so I ran an inverse Fourier transform on it so we could listen to it. I was pleasantly surprised to find that the result was a chiptune-y version of the original Bach piece.