I just came across this xkcd comic about Real programmers.
I started emacs invoked butterfly command and that's my first easter egg. Later I digged around and found several games & a few other easter eggs. Here are a few of them.
1. yow was removed in Emacs 22 due to copyright problems. But you can get it manually.
Download quotes file to your .emacs.d directory
M-! wget bit.ly/emacs-yow -O ~/.emacs.d/yow.txt.gz
Specify the path of file in Your init file
(setq yow-file "~/.emacs.d/yow.txt.gz")
2. To know about each of the easter eggs, you can use help
C-h f RET <function> RETMore articles about Emacs