Wikipedia is 60 times larger than the next largest encyclopedia (according to Wikipedia). The English version has between 4 and 5 million articles. Printed out, the English version would be contained in almost 2,000 Britannica-sized books.
It is the largest source of knowledge in human history, and it is almost entirely created by volunteers.
Wikipedia is the product of a collaboration between Jimmy Wales (pictured) and Larry Sanger, a philosophy professor. Both had loved Fredrick Hayek's Use of Knowledge in Society, which argued that knowledge is decentralized. While working on a for-profit encyclopedia, they were introduced to some of the concepts of "extreme" programming, particularly the idea of open source collaboration through software called "wikis."
The extreme programmers divided the world into two models. The first, the "Cathedral," are projects that are created by an exclusive group (a company, university professors). The second, and to them, far superior, is the "Bazaar," which are projects that are open-source. Oceans of optimistic ink have been spilled about the open source movement, and there have been some successes: wikipedia, reddit, and Linux are examples. There have also been many failures, often not discussed.
Why do some open source projects succeed while others fail? A forum like "Ask Historians" on reddit - where professional historians continually respond to any question with well-written, detailed explanations and cited sources, all for free - hints at the difference: there is a deep human desire to teach.