There are whole websites dedicated to the discovery of Easter Eggs but these are some of my all time favourites.
Some you may know but others are probably new to you.
The Skype Cat
In any skype chat window hold down the letters C A T and watch a pussy cat appear next to your name.
The Skype Pencil
In any skype chat window hold down the letters and watch as a pencil is broken next to your name.
The Skype Emoticon
Skype has more emoticons available to you than just the ones that it shows you. Try typing the following in any skype chat window.(finger) (bandit) (mooning) (swear) (drunk) (rock) (poolparty) (bug) (ninja) (smoking) (fubar) (tmi)
The Firefox Robot
In the address bar type about:robots and don't forget to click on the buttons
The Firefox Devil
In the address bar type about:mozilla
The Yahoo Yodel
Yahoo.com ran a very successful advertising campaign featuring a high pitched yodel. You can still hear it if you click on the ! on the logo at www.yahoo.com
The PHP pencil nose (or dog)
On any server running php 4 add ?=PHPE9568F36-D428-11d2-A769-00AA001ACF42 to any url. usualy you will see a picture of one of the PHP developers with pencils stuck up his nose but on April 1st you see his dog
Open Office Star Wars
Bored at work and the boss wont let you install games on your PC. Play Star Wars inside OpenOffice.org instead. Enter this formula in a cell =game() and then enter =GAME("StarWars")
Naked Joomla
Ever wanted to see the great joomla template designers naked?
You can do this on any Joomla site by appending ?template=naked to the url.