Back in 2004, someone pointed out to me a nice way of getting free CAPTCHA solvers by offering certain audiovisual materials to visitors and requiring the visit

Back in 2004, someone pointed out to me a nice way of getting free CAPTCHA solvers by offering certain audiovisual materials to visitors and requiring the visitors to solve CAPTCHAs in order to access those materials. To maximize effectiveness, said audiovisual materials tended to "appeal to the prurient interest," to put it politely.