Most people have posted my personal favourite Easter Eggs, apart from the ones from Saints Row 2, that game is hugely full of little inclusions.
At one point Birras and I were playing co-op and we found a barbershop quartet on the street, singing really well. They were an actual recording, playing on the street and eventually packed up and got mugged by some pimps, that we promptly executed.
I like that the devs just got in a barbershop quartet, recorded them, then stuck 'em in as a random encounter.
Plus, In MW2 Multiplayer we went searching for Easter Eggs and we've found a few. Twice we found coloured pictures of the IW Dev Team, we also found a golden train numbered "1337", plus in the "Estate" map it is discovered that in the CoD universe, Infinity Ward is a huge company.
They make a console called the "Gamestation 6" or something, which plays a few games. They also own DVDs and real books, like Frankenstein or 1984. Then in the basement they have movie posters with references to Left 4 Dead and Mass Effect, as one film is called "The Massey Effect" and another has in its tagline "left for dead".
In the Post-Credits Museum we noticed that Infinity Ward also sell retail, as in one of the backgrounds they have an IW "For Sale" sign. They also make cars and ammo, as every number plate and ammo crate has the letters IW on it somewhere.
EDIT : What the hell? This thread is from August, but I saw it on the front page? Sorry 'bout the necro, I thought it was recent.