All stock agencies offer some form of enhanced or extended license, in which buyers are given unlimited rights to reproduce photographs, typically over 250,000 times. In the case of Shutterstock the contributing photographer receives $28 instead of 0.25c/0.33c. Its always  a nice surprise when you sell one. In the last three years I have only sold five enhanced licenses compared with 3000 regular sales. Although I will probably never find out, Somewhere in the world these five pictures are being reproduced over 250,000 times.