Not all Easter eggs are made out of chocolate and covered in coloured foil.
By Mark Ward
BBC News Online technology correspondent
Some of them are made out of computer code.
Easter eggs unlock swearing clips on the Snatch DVD
Many programmers employed to create computer games and office software often add in hidden extras that are not mentioned in the manual.
Only when people come to use the programs, play the games and explore all the software's features do these secret add-ons become known.
Computer game programmers are the most prolific creators of Easter eggs and the concealed extras take many forms.
The simplest form of Easter egg is simply a cheat code that boosts the abilities, resources or finances of the faction or character the player controls in the game.
Typing in the code, usually a strange combination of keys or keypad presses, bestows the extra cash or abilities on the player. Lists of cheat codes for games are readily available online.
SAMPLE EGG SITES
Other Easter eggs are quirks of the computer system that arise simply because games are so complex. These eggs can be as simple as knowing where to jump to avoid taking damage when navigating terrain.
Other eggs of this type involve exploiting bugs in the code for material gain. By endlessly repeating a series of steps, or carrying out specific actions at certain points in a game, players find it possible to gain huge amounts of experience points, money or increases to abilities.
The hugely popular online game EverQuest recently almost fell victim to a bout of inflation because players created small in-game programs that turned small amounts of cash into bigger ones.
Yet another type of Easter eggs are made by programmers keen to leave their mark on their creation and typically involve adding incongruous props to the game world.
In Warcraft III many players were surprised to discover futuristic weapons from StarCraft, an earlier title created by Blizzard Studios, hidden in some sections of the fantasy game.
Cows are popular with game creators
In Quake III Arena some of the dismembered bodies strewn around parts of the games levels have the faces of games' designers. In Duke Nukem Forever some of the level designers left phone numbers and e-mail addresses scrawled as graffiti in some locations.
Programmers often add the Easter eggs to lighten what would otherwise be much more serious games.
For example, in Age of Mythology some of the Norse Herser units have very strange names such as Refreshingbeverageblender.
But the most complex type of Easter egg are those completely hidden until unlocked by a secret code.
Unbeknownst to many players, there are hidden artefacts and levels in many popular games.
In Diablo II, for example, by combining the correct items it was possible to enter a secret cow level, take on heavily armed farm animals and go up against the Cow King.
Cows are popular with games creators because killer flying cows can also be unleashed in Neverwinter Nights.
But it is not just computer and console games that contain Easter eggs. Some business programs have hidden extras too.
Fly while you do your accounts in Excel 97
Microsoft's Excel 97 was famous for having a crude flight simulator hidden inside it. The unpatched version of Excel 2000 also had a driving game hidden in it too.
As DVDs become more complex, Easter eggs are starting to turn up on the discs too.
On the Spider-Man DVD there are up to seven Easter eggs that unlock clips of out-takes, special effects shots from the film and contributors talking about some aspects of the movie.
On the DVD of the film Snatch, Easter eggs include a compilation of scenes in which characters swear.