There was a discussion about polymath projects some of it was described here.
There was an article in the Atlantic about Math Overflow. It mentions the Polymath project. It is available here.
In _the Best Writing on Mathematics: 2010_ there is an article “Massively collaborative mathematics” by Timothy Gowers and Michael Nielsen.
the book _Ramsey Theory: Yesterday, Today, and Tomorrow_ contains an article about Euclidean Ramsey theory “Open problems in Euclidean Ramsey theory” by Ronald Graham and Eric Tressler. it is available here.
Finally there is an article “Memories of Martin Gardner” which appeared in the Notices of the AMS, volume 58, Number 3. It is available here.