Here's a few easter eggs in Ubuntu I have found. If you know of more, feel free to mention them in the comments.

1. The Ubuntu cow

   The first easter egg can be found in a program called "moo". Open the terminal and type aptitude moo. The program will say "There are no Easter Eggs in this program", but don't trust everything you read.

   Now type aptitude -v moo. It will give you almost the same message as the previous one, but slightly different.

   Keep adding an extra v to the command until you reach aptitude -vvvvvv moo (6 v's). This will be the last easter egg in the program.

2. Funny Ubuntu names

   As if the names of the different Ubuntu releases weren't funny enough, the developers decided to give each Ubuntu release alternative names.

   Type either of the following in the terminal:

   zcat /usr/share/doc/linux-image-`uname -r`/changelog.Debian.gz | egrep -e "Release"
   or
   zgrep "The.*Release" /usr/share/doc/dpkg/changelog.Debian.gz

   And you'll get something similar to:

   The "Ben got a PowerBook for Christmas" Release.
   The "Quickest re-release ever" Release
   The "Oh crap, what did I get myself into?" Release.
   The "Welcome Ben!" Release.
   The "Morose Mungbean" Release.
   The "Laughing Lentil" Release.

3. Star-Wars in OpenOffice.org

   You thought office applications were all just meant for hard work. Well, think again. There's a hidden Star-Wars game in OpenOffice.org.

   Open the OpenOffice.org Calc application and enter =Game("StarWars") into any cell and press enter. This should launch a Star-Wars game which looks a lot like the old Space Invaders game from back in the Atari days.

4. Book of Mozilla

   This easter egg is not limited to just Ubuntu, but since Firefox is included with Ubuntu, I feel it has the right to be called an Ubuntu easter egg. Open a new tab in Firefox and type about:mozilla.

   The following message from "The Book of Mozilla" will be displayed: