EASTER EGGS #7 and 8 - Red vs. Blue
If you're a fan of the Halo-inspired machinima series Red vs. Blue, you'll definitely want to track these two Eggs down. The first is stupid easy - just stare closely at the tire of any Warthog in the game. Does that say "Puma"? As in, what Grif suggests calling the jeep because it looks more like that animal?
Even better is the secret conversation that takes place down a deserted hallway in the Crow's Nest mission. After you go through a door and see a Warthog explode in front of you, fight through the Grunts and continue forward, skipping the open doorway on the left. At the end of the passage, you'll witness two Marines arguing over a password. Depending on the difficulty you selected, the characters will be either Tucker and Doc, Simmons and Grif, or Church and Caboose... all of Red vs. Blue fame