A most noble and liberating specimen in hand-held electronics.
It used Bell Labs own spec. for in-band signalling to emulate a telephone operator's console and provide free calls and untracability to its posessor.
It is rumoured that the sale of these devices helped to fund the fledgling Apple Computer. Also notable: Apple's first modem, the cat something-or-other, was able to combine frequencies, via its dial-string, to produce CCITT tones.
See Also: Captain Crunch, Demon Dialer