Doom 3 includes several references to the original Doom:
- At the start of the game, the player can access a communal area where there is an arcade machine. The name of the game is "Super turbo turkey puncher 3". The game is formed mostly from Doom graphics, and features the fist, status bar face and a background which is clearly a screenshot from Doom.
- Throughout the game, magazines can be found in the Mars base. One of these, "GameHOG", features the status bar face blown up to fill the front cover.
- The original cheat codes, when typed at the console, will cause the message "your memory serves you well!" to be displayed.
- The light patterns in the pentagrams found in hell include a small image of the original status bar face.
- The pentagrams also feature the UAC logo.
- One of the rock images on site 3 features the box cover picture of the original Doom, slightly modified so that the player is carrying a Doom3-specific weapon.
- When sending a message to the fleet, the computer screen includes the code "IDSPISPOPD", one of the classic cheat codes.
- http://www.planetdoom.com/doom3/secrets/eastereggs.shtml - PlanetDOOM article on Doom 3 easter eggs
This is based on an article I wrote for the Doom Wiki.