The legend of the Yeti
The Yeti appears in a couple maps every patch. He is left there by the [Dev]s for us to find. You usually have to use "mpcheat ghost" to find him.
Further Reading: http://forum.americasarmy.com/viewtopic.php?t=41751
The Leavenworth Fish
If you find yourself in Leavenworth prison, either by shooting somebody in a training mission or by TKing on certain servers, you can enter into the console "mpcheat ghost" and ghost around and look at all the artwork in the other cells left by the Devs. In one of those cells is a fish swimming around with a laser-light attached to its head, a reference to the movie Austin Powers.
It is an unconfirmed rumour that if you shoot at civilian cars in maps such as Border long enough, or do enough damage to them, they will blow up.