Easter eggs are hidden surprises throughout the Call of Duty series. They are most prominent in Zombie Modes, in the games developed by Treyarch. Some are easy and can be achieved alone, while others require a great deal of cooperation. A lot of them, the most elaborate, can be used to gain an advantage either in a single player campaign or multiplayer zombies game, and some unlock achievements. Every Nazi Zombies map has a secret song which plays after activating a number of objects.
Note that map or level features that are distinctive or interesting, like the Porter Justice posters in the Modern Warfare games, the ghosts in the Zombie Mode maps of World at War, or the Brady Bunch house on Nuketown, are not noted here and can be found in the trivia pages of each map/level.