The types of Easter Egg archived on this website are known as Virtual Easter Eggs. This name is also known under several variations such as:
Computer Easter Egg - Virtual Easter Egg - Easter Egg (media) 
There is some debate as to standardizing this name. Wikipedia, for instance, has settled for 'Easter Egg (media)'. Here on Eggheaven, we use these terms interchangeably.
A Virtual Easter Egg is an obscure feature or messages hidden within some media products that are not immediately obvious to its users. These events can vary from simple animations - such as rolling credits of programmer names - to more elaborate features such as undocumented extras as found in some popular movie DVDs. Others eggs can be hidden tracks in music CDs (usually at the end of the disc) or extra features in games.
Nearly all are triggered by a set of commands or procedures carried out by the end user.
Try this easy example with Mozilla Firefox web browser:
Something like this will result:
Firefox About The Robots
The first computer Easter Egg discovered in a digital consumer product was most likely from a demo game created for the Fairchild Channel F - a ROM cartridge-based game console released by Fairchild Semiconductor in August 1976. The Egg was discovered in the Demo Cart supplied with the machine. At the end of the game, hitting buttons 1,3 and 4 at the same time then releasing them gave the programmer's name on screen .
Fairchild Channel F Game Console (1976)
Microsoft, historically was actively adding computer Easter eggs into some of its Windows based software, such as Dev/Spy Hunter in Excel 2000. They officially ceased this practice in 2002 at the time of the Trustworthy Computing Initiative. The last known substantial egg in a Microsoft product was the Matrix Effect in Microsoft Money 2000 (US & UK editions). Money 2001 featured a simple game of Reversi that would reveal the developers names.
Dev/Spy Hunter Game in Excel 2000
The Matrix in Microsoft Money 2000 BP
Currently, Virtual Easter Eggs are less common for new products, expect for a few notable exceptions. Now more likely found on DVD movie discs as this format represent a good way to build extras and interactivity, these though, are still declining. Google occasionally hides an egg into its services, as is seen with the Pac-Man egg of late.
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