Efficient Frontier, a search engine marketing firm, has used Mturk.com to accomplish tens of thousands of tasks since early 2006. Efficient Frontier helps companies figure out which keywords will bring Web surfers to their sites. With Mturk.com, Efficient Frontier can afford to pay three different people to look at each potential keyword, and vote whether those words are relevant to a given site. It costs the company just 4.5 cents to test each keyword, paying 1.5 cents to Amazon, and 1 cent each to three turkers.
It's certainly nothing for me, but it somehow attracts a lot of people. Is there something about breaking captchas in their terms of service?