Part of the charm of Emacs is its collection of easter eggs. It contains a tetris clone, Towers of Hanoi, Conway's game of life, and others.

One Easter egg I discovered by accident (through using smex) is:

M-x animate-birthday-present

Which takes an argument (the person's name) and displays an animation wishing that person happy birthday. My favorite Easter egg is gomoku. It is a fun 5-in-a-row game against the computer.