At one point in the 1960s, packets of the Cap'n Crunch breakfast cereal included a free gift: a small whistle that (by coincidence) generated a 2600 Hz tone. By dialing a telephone number, covering one of the whistle's two holes, and then blowing the whistle, it could fool the phone company into thinking the line was not being used while, in fact, the user was now free to make a call to any destination in the world. The phreaker Captain Crunch adopted his nickname from this whistle. 2600: The Hacker Quarterly got its name from the frequency itself.
en.wikipedia.org/wiki/2600_hertz