My hard drive speakers. These are 2 hard drives opened up with the read-write head connected up to a 10w stereo amplifier. Hard drives use electro-magnetics to move the read write head. The polarity of the current determines which way the head moves. In an audio signal the peaks and troughs are opposite polarities, meaning the head vibrates with an audio signal. This, combined with vibrating against the platters of the hard drive, make the sound.
I made a box out of acrylic for them and installed a volume control, 12vDC input, sound reactive neon strip and lots of LEDs (blue ones are flashing - the green ones on the HDDs flash because they are in series with the blue ones).