Blue box
The tone generator that mimicked the phone network's own control signals to take command of long-distance lines — Wozniak's first product, long before Apple.
A blue box emitted a 2600 Hz tone to free up a trunk line and then the multi-frequency (MF) tones the exchange itself used to route calls. With it a user could drive the telephone network like an operator and place long-distance calls with nothing billed. The boxes were built by hand from the 1960s onward.
Steve Wozniak designed and sold blue boxes together with Steve Jobs before they started Apple, and John Draper made the technique notorious. As with phreaking in general, the method ran out when the network's signalling was moved off the voice channel.