Macintosh 128
The Macintosh 128 was the original Mac, released on January 24, 1984. If you cracked it open, you could see signatures of the Macintosh Division at Apple on the inside case.

PC Network Icon
In Leopard, when a PC was spotted on the shared network, the Finder displayed a "Blue Screen of Death" icon.

emacs Therapist
Apple included the emacs Unix-based text editor in OS X, which is accessible from Terminal:
1. Open Terminal and type "emacs" followed by Enter.
2. Press esc, and type "x"
3. Type "doctor" and press enter.
4. Get help for your problems, without paying a ton of money.

Slow-mo Animations
It is a little-known secret that many of the beloved animations in OS X, including the "Genie" minimize animation, can be done in slow moetion. Just hold down the Shift key while you perform the action.

TextEdit Icon
Did you know that the icon for TextEdit contains the poem from Apple's Think Different ad campaign that ran during the late 1990s? The full poem can be found here.

Sosumi
The sound effect in System Preferences > Sound > Sound Effects called "Sosumi" has a rich history. A long time ago, The Beatles' record label, Apple Corps., filed suit over a name conflict. In System 7, Apple included this sound byte, named after the homophone of "so sue me." Read the dramatic history here.

Dashboard Weather Widget
During the Tiger keynote, Steve Jobs demonstrated the Dashboard Weather widget by showing all of the available weather conditions. Just hold down Option + Command key when clicking on the weather condition to see all.

Record User Picture in Lion
In Lion, users could now pick a vinyl record as their login icon. However, upon closer inspection, the record seems to have the words "Magic", "Revolution", "Boom", and "Unbelievable" as the song titles -- the words most often uttered by Steve Jobs during Apple keynotes.

Mail.app "Senders.tiff" File
In Mail version 10.4 (Tiger) and earlier, you could find the developer's pictures in a file called "Senders.tiff."

Knock, Knock
While Siri doesn't really like telling jokes, your Mac will greatly oblige. In fact, it knows a lot of Knock Knock jokes. Turn on Speakable Items in System Preferences > Speech > Speech Recognition. Hold down the Esc key and say "Tell me a Joke."