The monkey knut is a truly unique product. Described on their website as keychains and zipper grips, they can also be used as a defensive tool. A 3/4″ stainless steel ball bearing is wrapped in nylon parachute cord and hung ~6 inches from the keyring. I find myself spending more time spinning it on my finger more than fending off attackers, but if swung with force, the suspended ball-bearing could certainly cause bodily harm.
Mine is the original Knut Buster in Coyote Brown.($5.95)
(BTW: If you spin it on your hand, make sure that it doesn’t slip off and go flying across the room. Of course it has never happened to me. Why do you ask?)