ASCII art that creates an image out of characters is one of the great traditions left over from the teletype days. Vincent Chu just published a cool website that turns an image and a secret message into ASCII art. It relies on the fact that a number of letters have the same amount of ink in them. If the ASCII art needs a dense object, it could use an ampersand, a hash sign, or maybe a lower-case m. If they all have the same visual density, then the ASCII art generator could use them interchangeably. And if things are interchangeable, there's an opportunity for a hidden message.
Enjoy.