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.
Attempting to leave the game will give you a random message to go with "Press Y to Quit to dos" (Ex. Just leave. When you come back, I'll be waiting with a bat.- Don't leave yet - there's more Demons to toast!- You're trying to say you like DOS better than me, right?)
In released versions of Doom prior to V1.4, E1M4: Command Control contained a swastika design on the floor of one room. According to John Romero, it was placed there as a 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. The presence of a swastika would have caused Doom to be banned in Germany, and not just restricted to adults due to the game's graphic violence.
E4M1: Hell Beneath includes the letters "NIИ" in a part of the level, a reference to Nine Inch Nails, the American industrial rock band.  Also, 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.
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 Living
In the first level of the Doom II expansion: No Rest for the Living may be found a hidden message is written on the walls and ceiling of a room, which must be reached before they meet the 30 seconds after starting the game and push a switch.
The message reads "The castle was here," the word castle being a direct allusion to Russell "The Castle" Meakim, one of the game designers, employed by Nerve Software.
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 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.).
- In level MAP03: Main Engineering is a time when, on entering the second room, generating a huge pedestal where the blue key is triggered. However, if the player jumps inside the circle where it will create the pedestal after the key appears, you will be trapped in the machine unable to leave but does not die. The legend appears on the sides of the pedestal base "I suck at making maps" repeated on all four sides.
Plutonia 2 contains a hidden level, Chocolate, which cannot be accessed except by the level-warp cheat or the various source ports' console level-warp commands. Its name reflects its status as an Easter Egg.
Heretic: Shadow of the Serpent Riders
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 Heretic
- 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 when entered thrice in quick succession.
Rambo takes away all the player's weapons.
- The first hub's secret level Bright Crucible features a puzzle involving the Heart of D'Sparil. D'Sparil was the final boss in Heretic.
Hexen: Deathkings of the Dark Citadel
- In the Locus Requiescat level, the larger tombstones will give funny messages from the game developers if "used" by the player.
Strife - Quest for the Sigil
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.
Microsoft Excel 95
Pitfall: The Mayan Adventure
In Pitfall: The Mayan Adventure, typing IDDQD or IDKFA shows a picture of the Pitfall staff.
Wrack's loading screen features protips, which provide useful tips on playing the game. Some of them have references to Doom.
"To win the game you must kill me, John Ro- Oh, nevermind." - This refers to the reversed Icon of Sin chant from Doom 2.
"To kill enemies, shoot at them until they die." - Refers to the sarcastic Doom playing tip from a Gamepro magazine, "To kill the Cyberdemon, shoot at it until it dies!", which spawned the protip internet meme.
- Cheat codes
- References to Classic Doom in Doom 3
- Doom references in other games