Ask HN: What crowdsourcing as a service have you had success with?

I am looking to crowd source very basic activity, categorizing the image a person is looking at.

What I need is the fastest response as possible, and a large volume of work to be done (1000 jobs a day).

What are some crowdsourcing services you've used, what was the task, what was the cost per task, what was the response times per task?