An Easter egg is a message that is hidden or otherwise not immediately evident and is meant to amuse players and often used by the designers to refer to influences, development jokes, pet characters, previous works, or preferences.
In released versions of Doom prior to v1.4, E1M4: Command Control contained a swastika design on the floor of one room, meant as an homage to Wolfenstein 3D. Alpha versions of this level, dated April 2 and May 22, 1993, included the same room, but did not have the swastika design.  The swastika was removed in v1.4 of Doom. While the presence of a swastika would have caused Doom to be banned in Germany (where the display of Nazi iconography in video games is illegal, regardless of context), and not just restricted to adults due to the game's graphic violence, according to developer John Romero, the symbol was removed after an objection to its presence was sent to him by a veteran of World War II.
Most of the cheat codes in Doom contain or otherwise trigger easter eggs as part of their name or function:
- idchoppers references a game Choppers, created by a friend of Dave Taylor's for a state programming contest. The message "Doesn't suck - GM" is a reference to his friend's defensiveness over the game being "not quite finished" at the time of its release.
- iddt, besides consisting of Dave Taylor's initials, also causes those initials to appear as part of the player arrow on the automap.
- iddqd forms a reference to DQD, a supposed college fraternity called Delta-Q-Delta, for programmers which could only be joined by having at least one "Q", a common grade assigned for dropped courses, on one's transcript.
- idspispopd is a reference to the SPISPOPD phenomenon which arose on Usenet before Doom's release.
E4M1: Hell Beneath includes the letters "NIИ" in a secret area of the level, a reference to Nine Inch Nails, the American industrial rock band. This was included by designer American McGee, who was a fan of the band. id Software's next game, Quake, would feature sound and music by the band's frontman, Trent Reznor. Additionally, ammunition for the nail-gun and super nail-gun (a crate of nails) would be inscribed with "NIИ" on each of its faces. The easter egg is notably absent from the version of the level used by the Sony PlayStation and Sega Saturn ports.
In MAP30: Icon of Sin, after the player teleports into the huge room with the head of the final boss, a strange, unintelligible noise is played. This is stored in the Doom2 wad file as DSBOSSIT, and if reversed it becomes a voice saying "To win the game, you must kill me, John Romero". The voice is shifted down in pitch and is that of John Romero himself.
In the same level, John Romero's severed head on a stick features as the main boss creature, although the head is hidden behind the face of the final boss, and can only be approached with the noclip cheat. The player must indeed kill it in order to win the game.
The secret maps, MAP31: Wolfenstein and MAP32: Grosse, resemble E1M1 and E1M9 from Wolfenstein 3D. Both levels include Wolfenstein textures and SS Nazis that attack the player. In MAP31, SS Nazis take the place of soldiers and Demons are found in place of the dogs.
A Cyberdemon is in the place of Hans Grosse in MAP32; the name of this map also comes from Hans Grosse's name.
In the last room in MAP32 there are four Commander Keens hanging on ropes. They must be killed in order to exit the level.
At the beginning of MAP32, the message "You'd better blaze through this one!" is displayed; Billy Blaze is Commander Keen's real name.
No Rest for the LivingEdit
In the Doom II expansion No Rest for the Living, first released as part of the XBLA version in 2010, the first map contains a secret message "THE CASTLE WAS HERE" which is almost impossible to find under normal circumstances, as it becomes invisible 30 seconds after the beginning of the level. Directions on how to find it can be found here. It is a reference to the map's author, Russell Meakim (TheCastle).
There is a hidden room in MAP17: Processing Area next to the room containing the blue key that can only be accessed by using the noclip cheat. Several Demons are inside and will teleport out once the key is collected. In this room the player can see "Tom" and "NIИ" in large, glowing blue letters on the north and south walls respectively.
In the Super NES port of the game, at the bottom of the Knee-Deep in the Dead sky texture is a message reading, "Randy Linden ♥ Jodi Harvey." This can only be seen if the player looks inside the ROM's graphical resources, or uses a Pro Action Replay code that allows walking through walls in areas where the sky is visible.
Within the ROM itself, a hidden message can be found which reads, "Rage/Reality Engine written by Randy Linden. Special thanks to my loving wife, Jodi Harvey." It can only be found if the ROM file is opened in a hex editor, and is found at position 10E (270 in decimal).
Several oddities in the Sony PlayStation port of Doom are actually easter eggs, left by the Williams designers:
- In MAP57: The Marshes, a deadly trap area with an inescapable crusher is formed out of the letters "re", which are the initials of the map's designer Randy Estrella.
- The song "Breath Of Horror," first occurring on MAP09, contains what sounds like demons chanting or singing in low-pitched voices. If sped up 300%, the song seems to have the lyrics "Danny won. Danny won, hey." This may be a reference by Aubrey Hodges to coworker Danny Lewis (Technoman), and seems to be accompanied by the sounds of an ongoing ping pong game.
- On the secret level MAP59: Club Doom, sectors forming the letters WDT and JRS are found far to the south-east (bottom right) of the playable area of the map. These are the initials of the play testers for the game, which included Will Shen, Danny Lewis, Tim Heydelaar, John Stookey, Randy Estrella, and Steve Kramer.
In Doom 64, if a player dies and waits several seconds, messages will start to appear at the top of the screen every once in a while, usually taunting the player (e.g. "HAHAHAHAHAHAH!!!!", "LOOK AT THOSE DEMON FEET!", etc.).
Heretic: Shadow of the Serpent RidersEdit
Attempting to use some of the cheat codes from Doom and Doom II in Heretic has negative effects on the player:
- iddqd results in instant death with the message "trying to cheat, eh? Now you DIE!"
- idkfa takes away all the player's weapons, except the staff with the message "Cheater - you don't deserve weapons!"
Hexen: Beyond HereticEdit
- Like the Heretic Easter egg, in the demo version of Hexen the cheat codes from Heretic in Hexen has negative effects on the player:
- quicken results in instant death.
- rambo takes away all the player's weapons.
Hexen: Deathkings of the Dark CitadelEdit
- In the Locus Requiescat level, the larger tombstones which have the game developers' names written on them will give funny messages from the developers if "used" by the player.
Strife: Quest for the SigilEdit
There is a sound wave in VOICES.WAD named SUR4A1 that sounds like a bunch of people yelling "surprise!!".
Chex Quest contains many levels leftover from the original 3-episode Doom albeit trimming to 5 levels.
The texture CEMPOIS has a blackboard and in the bottom right-hand corner of said board, is the equation: √2 = chuck. Charles Jacobi was the Art Director/Lead Artist for Chex Quest.
A secret room in E1M2 contains pictures of all the people that worked on Chex Quest.
The last level contains the id PDA which contains special messages from the developers of the game. Immediately before entering the room with the Cyberdemon, there is a button marked with the id Software logo. Pressing this opens a door to a secret room containing the PDA.
Microsoft Excel 95Edit
In Microsoft's Excel 95 spreadsheet program (included in the Microsoft Office 95 office suite), there is an Easter egg known as the Hall of Tortured Souls, a three dimensional raycasting game engine created as an homage to Doom.  The existence of this Easter egg is one of the results of the "religion" said to have arisen on the Microsoft campus in the wake of Doom's release. 
Pitfall: The Mayan AdventureEdit
In Pitfall: The Mayan Adventure, typing iddqd or idkfa shows a picture of the Pitfall staff.
- IGN (10 December 2013). "We Play Doom With John Romero." YouTube. Retrieved 18 September 2015.
- Kushner, David. Masters of Doom: How Two Guys Created an Empire and Transformed Pop Culture. Random House Publishing Group, 2003. ISBN 0-375-50524-5
- The E1M4 swastika, from Lee Killough's site (archived)
- History behind the Romero's head Easter egg (archived)
- Doomworld forums , thread at the
- Wikipedia article on Easter eggs in computer programs and other media