Yes, Mechanical Turk is another platform that enables human computation work. We looked a lot at that in our early research. It does not, however, implement any internal quality control mechanisms, and it also only allows payment in US dollars, Indian rupees, or via Amazon gift card. The interface is also clunky and difficult for people with limited computer/internet experience to understand (too many windows within windows, basically).

So there are a number of reasons to sidestep Mechanical Turk.