Amazon has a very interesting service called "Mechanical Turk".
Mechanical Turk is a Crowdsourcing Marketplace for work. You can offer work to be done by others (paying for it) or you can offer your services (and be reward by it).
The offered taks are ones that can only be made by humans (HIT = Human Intelligence Tasks). The big advantage of using this service is the massive laboral capacity that it offers and reduced cost.
I rethought about this service when I saw a presentation by Aaron Koblin at the OFFF festival. Aaron put up a task on Mechanical Turk offering 2 cents to people that draw him a sheep. He gathered 10,000 drawings of sheep, made a book and a website available at http://www.thesheepmarket.com/. Besides the idea beeing funny and allowing him to win some money by reselling the drawings, it shows the capacity of the Mechanical Turk service.
Mechanical Turk puts at our disposal a very ease way to use a huge humanal laboral capacity.
Here is a video of Aaron presenting the project, and also some statístics about it.