The file
Two thousand rubber strands tied at the center into a ball that couldn't hurt you and didn't bounce. It was invented so small kids could catch, and adopted by every desk and locker in the country.
Rosie O'Donnell launched them into her studio audience for years, which is where most people saw the most Koosh balls at once. They're still around, and they still make the sound.
Sources: Wikipedia — Koosh ball