Image via WikipediaThe idea behind crowdsourcing is that the answer to a question is often more likely to be correct if you average the answers from a large number of non-experts rather than a single expert in the field. The term "crowdsourcing" has also been used for projects that outsource repetitive or challenging work to a crowd via the internet.
I have been thinking of outsourcing the problem of conversion of embedded phylogenies in PDFs back to newick/nexus format and have been looking at various science projects that have used crowdsourcing.
The most impressive from my point of view is Galaxy Zoo which has already resulted in a number of publications and impressive discoveries. Astrophysicist use the crowd to categorise 1000s of galaxies and have expanded the crowd tasks to include matching images of galaxies with randomly simulated images.
Stardust@Home is another astrophysics project which asks that the crowd looks through images for dust particles brought back to earth by a spacecraft in 2006.
Another cool project is the Open Dinosaur Project which asks that the crowd aggregates published measurements of dinosaur limb bones for many different taxa from the literature and directly measured from specimens to study the evolutionary transitions from bipedality to quadrupedality.
Foldit is a computer game enabling the crowd to contribute to our understanding of how protein folds. Figuring out which of the many, many possible structures is the best one is regarded as one of the hardest problems in biology today and current methods take a lot of money and time, even for computers. The idea of using human's spare time to get further insight is genius!
Another game that might not be directly relevant to science is Google Image Labeler which I found rather addictive. Google gets users to label/tag images as a side-effect of playing a game and this is probably used to improve image searches on the web. I list it hear because I came across a few images of animals that in some cases were labeled down to the latin binomial.
UPDATE: An interesting new crowd sourcing project at http://www.oldweather.org/ to help gather information about past climates from hand written nautical records.