Wikipedia, a project to produce a free content encyclopedia that can be edited by anyone, formally began on 15 January 2001 as a complement to the similar, but expert-written, Nupedia project. It quickly replaced Nupedia, growing to become a large global project. As of 2007, it includes millions of articles and pages worldwide, and content from hundreds of thousands of contributors.
The concept of gathering all of the world's knowledge in a single place goes back to the ancient Library of Alexandria and Pergamon, but the modern concept of a general purpose, widely distributed, printed encyclopedia dates from shortly before Denis Diderot and the 18th century encyclopedists. The idea of using automated machinery beyond the printing press to build a more useful encyclopedia can be traced to H. G. Wells' book of essays World Brain (1937) and Vannevar Bush's future vision of the microfilm based Memex in As We May Think (1945). Another milestone was Ted Nelson's Project Xanadu in 1960.
With the development of the web, many people attempted to develop Internet encyclopedia projects. Free software exponent Richard Stallman described the usefulness of a "Free Universal Encyclopedia and Learning Resource" in 1999. He described Wikipedia's formation as "exciting news" and his Free Software Foundation encourages people "to visit and contribute to the site". One never-realized predecessor was the Interpedia, which Robert McHenry has linked conceptually to Wikipedia.