Easter Eggs, when related to software, take usually the form of hidden content put there by the developers. More info here: http://en.wikipedia.org/wiki/Easter_egg_%28media%29
You'll see it's very usual ;)
In our case it goes from hidden messages put in textures to some content which were not originally part of a car. Most of them are put there just to make you laugh or to dedicate something to somebody (usually to a team member).
Cars that i recall got a Easter Egg are the Porsche 904, Aston Martin DB4, Abarth TCR, almost all the big TransAm, Austin Healey Sprite, etc.
I think it will be good to create a separate thread just to see how many of them you're able to find