One of these "eggs" is the ability to play Tetris in Terminal. Here's how to do it. First, open a new Terminal.app (/Applications/Utilities) window and type "emacs," then hit enter. After the screen loads, press "escape," then "x." Type "tetris" (lower-case) and hit enter. There you have it, a fully playable game of Tetris. Use the arrow keys to control the blocks.
Want more tips and tricks like this? Visit TUAW's Mac 101 and Terminal Tips sections.