Easter Eggs

Easter Eggs are items or other things that are intentionally placed in the game by the developer. Bungie has placed many Easter Eggs in the Halo games. Most of them are in some way related to their earlier game, Marathon.

Halo: Combat Evolved
See: Halo: Combat Evolved Easter Eggs 

Halo 2
See: Halo 2 Easter Eggs 

Halo 3
See: Halo 3 Easter Eggs