The monorail is great for spotting! Furthermore, the monorail allows one to view aircraft from each of the passsenger terminals; when you get tired of one, simply hop aboard and go to the next terminal. (This also allows you to leave your car in one lot and tour Terminals A, B, and C.) I don't know of any other places where one can pull over and take pictures. Security is pretty tight, with tall chain-link fences and patrols, and the traffic patterns around the airport are a nightmare. In the late afternoon and early evening, you will see a huge variety of airlines. In September, check out the airliners collectables (a.k.a. mania) show at the Newark Mariott.
The aeroplane has unveiled for us the true face of the earth. -Antoine de Saint-Exupery