Truck Nuts are an ornament for your car, ostensibly for a truck. They manage to be both odd and crude, and for the faint of heart, they manage to move all the way to disturbing. They have become something of a cultural phenomenon in the USA this year (2002). They are imitation testicles, and are hung from someplace in the rear of the car; Popular mounting locations include the trailer hitch and rear bumper.
It is fair to mention that "nut" is not only slang for "testicle", but also a synonym for "fan" (atic). However, this writeup is about testicles (of a sort), not truck owners.
Plastic "Truck-Nuts" (the original brand) come in eight colors; there are brass ("Brass Ballz") and steel ("Nutz of Steel") Truck Nuts as well. They're about eight inches long and five inches wide, and they have a fair amount of detail. In addition, "The rubber ones are made of a thick hard vulcanized rubber that can take a beating."  They have a hole at the top to facilitate mounting; or perhaps I should say hanging.
There are of course various imitators; a company called "Wonderfully Wacky" makes a much less detailed (and smaller) pair in red or blue plastic, and "American Brass Balls" sells "Brass Bumper Balls", which are fairly close to 1:1 scale -- for a human.
- Website: Truck-Nuts (http://www.truck-nuts.com/)