The following is a list of references to other games and media found in Wolfenstein: The New Order:
While Bobby Bram drives Blazkowicz to his objective in London during the opening sequence of Chapter 6 "London Nautica", notice that the keychain hanging from the ignition is a rocket launcher from Quake 3.
Super Mario Bros.Edit
Go into your hideout and head upstairs to Max Hass's bedroom to find Power Stars plushies on his bed from the Mario series.
During Chapter 7, Anya will ask for a sample of the moldy "Super Concrete". Look to the right of the Kreisau Circle Hangar entrance and search for a big red circular door covered in steam. It is marked with "101", a reference to Vault 101 and its large circular door in Fallout 3.
The "Nightmare" ("Escape from Castle Wolfenstein") is a bonus level based on the first level of Wolfenstein 3-D. To access it, play Chapter 4 and meet Anya in the Resistance secret base. After the helicopter theft, head upstairs to the top floor and find the room with an old mattress on the floor near the window. Use the bed and choose the option to enter a nightmare.
In Fergus' Timeline, you'll see the letters IDKFA written on a piece of wood in Tekla's room. This is the famous cheat code from Doom where it'll give you full health, ammo and armor as well as all keys and weapons in the game.