Easter Eggs can be hidden treats, homages, trivia, or anything developers wish to hide, with the intention of being found, within a game. The placement of an Easter Egg is typically such that they usually take some hunting to find, hence the name.
Easter Eggs have appeared in games for literally decades. Developers simply can't help but put in references to their favorite things and people, including themselves.
At the time of Adventure's release, Atari was not in the practice of crediting its developers with the work they produced. Thus, Warren Robinett, the developer of Adventure, did it himself by hiding his name and title within the game for players to find. This serves as one of the first known Easter Eggs in gaming.
Hidden within the very first level of the game was the Rave Gun, a weapon which fired balls of powerful energy in bursts of color which flashed across the screen in time to a pounding trance techno beat.
Doom II: Hell on Earth - 1994 - PC
Doom II: Hell on Earth actually contained several easter eggs, including the severed head of Producer/Programmer/Level Designer John Romero on a stick. Oddly enough, the head also functioned as the final boss' "weak point" in the game. Additionally, the player could access two extra levels not included in the normal level progression of the game. These levels were recreations of two levels from id Software's famous Wolfenstein 3-D, and to beat the second level, players had to murder four representations of Commander Keen.
Among the many easter eggs players with high luck stats can stumble across while wandering between the game's areas are, a large Godzilla-like foot print with a corpse inside, a used car dealership and a crashed U.F.O containing a ray gun and a photo of Elvis.
One of several easter eggs found within Grand Theft Auto IV is within the Statue of Happiness (Liberty City's version of the Statue of Liberty), where the player can find a huge beating heart, dubbed the "Heart of the City".
An actual easter egg was discovered close to a helipad, where the player had to get to the helipad in order to jump to the building and fall through the window. The easter egg read 'Happy Easter'.
At the end of Chapter 3: Part 8, the player can see the godly armor and Blades of Chaos used by Kratos in the God of War franchise.
Halo: Combat Evolved - 2001 - Xbox, PC
As with all subsequent games in the franchise, Bungie filled Halo with plenty of secrets and easter eggs, including cryptic symbols on weapons, secret music tracks, and a Grunt that explains his desire for the food nipple. And several Marathon icons could be seen, from Bungie's previous game series. Marathon icons can be found somewhere in all Halo games.
Halo 2 - 2004 - Xbox, PC
In several multiplayer levels, the developers added photos of themselves: they were hidden in tough to access parts of the level, or even in the skybox. Also skulls were hidden in each level of the campaign, a special effect was granted upon finding them and lasted until the console was shut down. Just like other Halo games, Marathon icons could be found in several places.
When playing as Fox or Falco on either Venom or Corneria you can activate a special taunt by rapidly pressing left and right on the D-pad. When this taunt is activated your character will have a special conversation with other members of the Star Fox crew. If you look on the bottom of the Barrel Cannon trophy in the viewer you can see a message that reads "2L84Me" .
Utilizing Solid Snake's taunt on the Shadow Moses stage, players can listen to various, often humorous radio conversations between Snake and and various Metal Gear Solid characters about Snake's current opponents. When the Pokemon Stadium 2 stage transforms into its ice form if you look inside the shack in the middle there is a picture of a white cat, it belongs to the designer Masahiro Sakurai.
Before the launch of the third Zelda game there was a contest in Nintendo Power to own a room in the game. The winner was Chris Houlihan and his special room is notoriously hard to access and involves the need of using the Pegasus Boots. There are forty-five blue Rupees in the room and a plaque that reads "My name is Chris Houlihan. This is my top secret room. Keep it between us, OK?"
A number of easter eggs were added to Titan Quest via the Immortal Throne expansion. The largest was a secret cave that could be found on the beach near the beginning of the fourth act. The cave had a locked door baring entry that would only open if the player was carrying an otherwise useless item called The Overlord. The Overlord was a small turtle shaped object (a nod to the number of oddly placed turtles that could be found throughout the entire game) which could only be dropped by the final boss, Hades. Once the player gain entry to the cave they would be faced by a wide variety of powerful monsters that looked like creatures from every act of the game but typically had very different ,or vastly exaggerated, powers. Amusingly, each of these creatures were named after members of the development team. Other smaller easter eggs included small scrolls marked by the symbol of a turtle which could be found on rare occasions in normal loot drops. These scrolls had writing on them that varied from simple jokes, such as a letter from a satyr in the beastman army to his loved ones, to serious poetry.