The Mechanical Turk Requester FAQ has a lot of worthwhile information inside.
ZapThink writes about The Human in the Machine.
ClickWorkers is a task-specific tool, similar in a few aspects to Amazon's Mechanical Turk. They say "There are many scientific tasks that require human perception and common sense, but may not require a lot of scientific training. Identifying craters on Mars is something almost anyone can do, and classifying them by age is only a little harder."