What are some alternatives to Amazon Mechanical Turk?

What is Amazon Mechanical Turk and what are its top alternatives?

Amazon Mechanical Turk is a marketplace for work that requires human intelligence. The Mechanical Turk web service enables companies to programmatically access this marketplace and a diverse, on-demand workforce. Developers can leverage this service to build human intelligence directly into their applications.

Amazon Mechanical Turk is a tool in the Crowdsourced Labor category of a tech stack.

Top Alternatives to Amazon Mechanical Turk

CrowdFlower
CrowdFlower is the world's leading crowdsourcing service, with over 800 million tasks submitted by over four million contributors. We specialize in microtasking: distributing small, discrete tasks to many online contributors, assembly-line fashion - for instance, using people to check hundreds of thousands of photos every day for obscene content. ...