For decades, programmers have hidden secret features and surprises inside the software they write. Such tidbits are known as Easter eggs, after the annual holiday hunt. (Note that undiscovered hard-boiled eggs may eventually create less pleasant surprises if left to mature in warm places.) As you're enjoying the Easter holiday, we've got a few links for Apple's Easter eggs.
For OS X Easter eggs, the Easter Egg Archive has a fairly comprehensive list that includes some classics; the BSOD Windows icon and the "here's to the crazy ones" copy on TextEdit's icon are particular favorites.
The classic Mac OS was populated with plenty of Easter eggs -- even inside the hardware itself -- but did you know that there was even an Easter egg in Inside Macintosh, the developer documentation for the Mac? Folklore.org has the story.
As OS X is built atop the BSD flavor of UNIX, it carried forward some truly ancient Easter eggs from the older operating system. The command-line calendar program's data files include some mythological/fictional anniversaries, including a disputed birthday list from Tolkien's The Lord of the Rings.
Apple's newest (and most portable) OS is no stranger to the Easter egg tradition, but the new hidden items have a speakable twist. Siri's subtle movie reviews and silly answers to simple questions have taken over for some of the spontaneous/secret items you might find on the Mac.