Julie Rehmeyer of Science News has what I think is one of the best versions of the story of the Polymath Project -- mathematician Tim Gowers's successful experiment to solve a non-trivial mathematical problem using online collaboration through his blog (and, later, a wiki).
[In January 2009,] University of Cambridge mathematician Tim Gowers decided to run a little experiment. Was it possible, he wondered, for a large number of mathematicians to collaborate openly on the Internet, pooling their ideas around a single problem? If it were possible, would it be easier, more efficient, more fun? Could the mathematicians together solve a problem they might not be able to solve individually?
So he posed a problem on his blog, one that was important and challenging and that lots of people had already worked on, including himself. [...]