2600 Hz is the frequency in hertz (cycles per second) that AT&T formerly put as a steady signal on any long-distance telephone line that was not currently in use.
At one point in the 1960s, packets of the Cap'n Crunch breakfast cereal included a free premium: a small whistle that (by coincidence) generated a 2600 hertz signal. By dialing a number and then blowing the whistle, you could fool the phone company into thinking the line was not being used while, in fact, you were now free to make a call to any destination in the world. The hacker Captain Crunch adopted his nickname from this whistle.
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2600 is also a Hacker magazine.