"Every program attempts to expand until it can read mail.
Those programs which cannot so expand are replaced by ones which
can." Coined by Jamie Zawinski (who called it the "Law of
Software Envelopment") 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). It is commonly cited, though with widely varying
degrees of accuracy.