A Tesla coil is a type of resonant transformer, named after its inventor, Nikola Tesla. Tesla coils consist of two, or sometimes three, coupled resonant electric circuits and are used to produce high voltages at high frequencies. Voltages can get to be well above 1,000,000 volts and are discharged in the form of electrical arcs.
Tesla Downunder is an Australian site detailing almost 200 high voltage and other projects. The site shows some amazing photos.