“Every program attempts to expand until it can read mail. Those programs which cannot so expand are replaced by ones which can.”
— Zawinski’s Law
Coined by Jamie Zawinski, a engineer on an early release of Netscape navigator, to express his belief that all truly useful programs experience pressure to evolve into toolkits and application platforms (the mailer thing, he says, is just a side effect of that). Examples of this law are Emacs and, of course, Netscape/Mozilla.