HD Speaker.

This is what happens when you have too much time, and a dead hard drive lying around. This thing is easy to make, all you need to do is hook up an amplified audio signal to the two wires of the coil that controls the arm of the hard drive.

A really fun extension of this project that I've seen, is to mount a laser pointer and use some old CD bits as mirrors to make an oscilloscope. Details of the extension can be found here.