It's safe to say that Mechanical Turk, the new API from Amazon, is one of the most unusual, ingenious services created thus far. And if you don't get it at first you're not alone, it can take a bit of time to digest and get your head around (the name is a reference to this hoax).
As Phil Wainewright points out in Wetware as a service and Fine-grained tail of the Mechanical Turk that "in essence what it does is provide a programmatic interface to the individual brainpower of Amazon's registered users. Developers who need a program to perform an action that computers don't do very well — for example, writing descriptions or evaluating images — can instead have their program hand off the task to the Mechanical Turk, pay a fee, and just wait for the answer to come back. The Amazon service takes care of advertising the task to users and paying them on successful completion."
What to use it for? Very small tasks for one. For example, one of the initial 'Requesters' using the service is Amazon's own A9 mapping folks who are offering a few pennies each time you review a set of photographs to verify which of them best corresponds to a given business or address (ex: "Which of these five photos best shows the Allstate Insurance office at 500 9th Avenue?"). For a human to do this typically takes a few seconds but for a computer this is simply impractical. Amazon can leverage their entire marketplace infrastructure to manage this all the way down to the micropayments. As many have noted, it's very long tail and Web2.0.
And in another bold move, contrary to nearly all the other web services out there, this is not an API that gives programmatic access to something you could otherwise get through a GUI: you can only use this service through these APIs. Of course this may change in the future, but reflects how much Amazon is really providing a raw commerce infrastructure, not your traditional online experience. Like the web as platform.
More good details at: Slashdot, Google Blogoscoped, Greg Yardley, Greg Linden, and Richard MacManus Top Ten Web 2.0 Problems Amazon Mechanical Turk Can Solve For Me.