After a few months of tossing this idea around in my head I recently constructed the Multichord, a single-stringed musical instrument that uses a hard drive to vary the tension in the string. It can play a full octave of notes by applying specific tensions related to the voltage applied to the hard drive's read-write head, as produced by an array of power resistors. A musician sets the note with pairs of buttons on the end of the instruments, then plucks the string to make the note.
I have detailed the project with photos, a video, and plans and schematics below. Check out the video below and click through to the Project Page for all of the information.