Really, really big sparks. The lightning stroke is especially wowsome.
This page of Bert Hickman Teslamania site shows what happens when electricity really gets out of hand. I usually think of electric as being well-behaved stuff that hangs around in wires and doesn't do anything out of the ordinary unless I do something plain stupid to it first. I am not correct in this assumption.
This page reminds me that electricity is actually made out of magic, and although some people know enough about things like capacitance, loading and "three phase" to stuff it into the wires in the first place, electricity's nature is more akin to demons and chupacabras than ordinary stuff.