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An undocumented feature of an application or other program is called an Easter Egg, called such because it is hidden, but usually cool to find. An example is this web page, which you get to by clicking on the Macintosh logo, even though it's not mentioned anywhere to do so.
Programmers install such Easter Eggs into their programs as in-jokes or as bonuses for people to find. Easter Eggs are generally fun and/or harmless, unlike computer viruses which, although hidden, are malevolent.
Easter Eggs are usually activated by entering a password or performing a certain combination of keystrokes at a certain point in a program. Games which run on dedicated machines such as PlayStation or Nintendo are usually full of such eggs (e.g. "secret moves" in fighting games.)
Some actual examples of Easter Eggs are:
Drop me (Chris Fernandes) a line if you find this page. I'd be interested in hearing about any Easter Eggs you may have run across. I'll add them to this list.