For as long as software has been created, geeks have embedded hidden nuggets of unexpected functionality into their software.
Some programmers create them to show their friends for a quick laugh while other programmers create them for fan service, and still others create them just to quietly rebel.
My first experience with an easter egg was in the original Super Mario Brothers with the minus world that is accessed by jumping backwards through the bricks in world 1-2. Technically speaking, this is actually a glitch in the game and not an easter egg in the truest sense of the meaning.
By the truest sense of the meaning, the first easter egg that I can remember finding with some help from a fledgeling Internet community was the cow level in Diablo 2.
The novelty and fun factor that those types of easter eggs bring to a piece of software has always appealled to me and I've always done my best to add one to every major project that I'm involved with.
One of my favourite methods to uncover an easter egg is when developers use the now famous Konami code to invoke an exciting new experience burried deep within their software.
Finding an easter egg is often considered a right of passage for true die hard fans of a software program or website. Putting one in is an indicator of the connection that programmers have with their product.
In any case - I think it's cool. Have a look at some of the more famous easter eggs out there on The Easter Egg Archive.