Ah, the exquisite noise of Clackers! We could listen to it for hours--or until our mother took it away from us. Then we had to play with Dee Dee's down the street.
Stolen from www.toynfo.com:
It was essentially two large marbles attached by a sturdy string with a ring in the center. You would put your finger in the ring allowing the marbles or balls to hang below. Here is where the fun starts. The idea was to get the two balls tapping (or clacking if you prefer) against each other by pulling up on the ring lightly. Once you got the hang of it, you could get them going faster and faster until they were smacking each other above and below your hand in a stunning arc.
These toys are now legendary. They started finding their way into the schoolyards in the early seventies and in 1971 were yanked from shelves because kids were suffering eye injuries when the fragile, glass-like balls were shattering from the violence of contact with their mates. I believe a plastic version may have been released after the original and the more menacing version was pulled, but I don't think that it enjoyed the same popularity.
****Here are the Romper Stompers--another favorite toy-- Collin referred to in the comments: