I've always been fascinated by Tesla Coils. The operating principle is so elegant - you set up one circuit (your power supply) that oscillates at some high frequency, and you set up another circuit that uses the earth to resonate with your power circuit. Energy is transferred and converted from current to voltage, and the end result is man-made lightning - an incredible display of light and sound.
Newer versions of the Tesla coil allow the control of the resonant frequency. Have you ever noticed the sound that a flourescent light makes? That kind of "buzz" ? You're hearing the frequency of the power that is coming into your house (around 60Hz). When you control the resonant frequency of the Tesla, you control the tone that it generates, and thus the Singing Tesla Coil is born. This is not lightning set to music - the sound is generated by the spark itself.
The band ArcAttack has integrated a pair of singing Tesla coils into their repertoire, with epic results: