The idea to build a free encyclopedia using the Internet can be traced at least to the 1993 Interpedia proposal; it was planned as an encyclopedia on the Internet to which everyone could contribute materials. The project never left the planning stage and it was overtaken by the explosion of the World Wide Web, the emergence of high-quality search engines, and the conversion of existing material.

Digitization of old content

A key branch of this activity is the digitization of old printed encyclopedias. In January 1995, Project Gutenberg started to publish the ASCII text of the Encyclopædia Britannica, 11th edition (1911), but disagreement about the methods halted the work after the first volume. For trademark reasons this has been published as the Gutenberg Encyclopedia. In 2002, ASCII text of all 28 volumes was published on http://1911encyclopedia.org/ by another source; a copyright claim was added to the materials, but it probably has no legal validity. Project Gutenberg has restarted work on digitising and proofreading this encyclopedia; as of June 2005 it had not yet been published. Meanwhile, in the face of competition from rivals such as Encarta, the latest Britannica was digitized by its publishers, and sold first as a CD-ROM and later as an online service. Other digitization projects have made progress on other titles. One example is Easton's Bible Dictionary (1897) digitized by the Christian Classics Ethereal Library. Probably the most important and successful digitization of an encyclopedia was the Bartleby Project's online adaptation of the Columbia Encyclopedia, Sixth Edition, 2000, which was put online at http://www.bartleby.com/65/ in early 2000 and is updated periodically.

Creation of new content

A further related branch of activity is the creation of new, free contents on a volunteer basis. In 1991, the participants of the Usenet newsgroup alt.fan.douglas-adams started a project to produce a real version of The Hitchhiker's Guide to the Galaxy, a fictional encyclopedia used in the works of Douglas Adams. It became known as Project Galactic Guide. Although it originally aimed to contain only real, factual articles, policy was changed to allow and encourage semi-real and unreal articles as well. Project Galactic Guide contains over 1700 articles, but no new articles have been added since 2000; this is probably partly due to the founding of h2g2, a more official project along similar lines.

Encyclopedias that are no longer online

Interpedia

Nupedia, a slow-moving project to produce a free peer reviewed encyclopedia. Nupedia shut down on September 26, 2003, and much of its content has since been assimilated by Wikipedia.

GNUPedia, an initiative which did not come to fruition.

Currently online encyclopedias

All info about.com - a collection of about 100 separate sites on a wide variety of topics written by experts

Baidu Baike - a Chinese collaborative online encyclopedia hosted by the major Chinese search engine Baidu

Citizendium - a new project opened in March 2007 by Larry Sanger

Conservapedia - a project to create an encyclopedia written from a viewpoint supportive of Conservative Christianity and Young Earth creationism

Ek$i Sözlük - a collaborative hypertext dictionary in Turkish.

Enciclopedia Libre - a Spanish fork of the Spanish Wikipedia, using wiki software, released under the GFDL.

Everything2 - has a wider range and does not exclusively centre on building an encyclopedia; its contents are not available under a copyleft license

h2g2 - a collection of sometimes humorous encyclopedia articles, based on an idea from Douglas Adams's The Hitchhiker's Guide to the Galaxy. Articles are not freely redistributable.

Net Sözlük - a categorized in Collaborative Hypertext Dictionary, is a web page including humorous descriptions for various words, terms and even sentences.

Open Site - Internet encyclopedia founded in an effort to build a free categorized community-built encyclopedia, inspired by the Open Directory Project.

Project Galactic Guide - inspired by The Hitchhiker's Guide to the Galaxy, was one of the first encyclopedia projects

Private Sözlük - a collaborative hypertext Turkish dictionary and encyclopedia site that shares a similar concept with Everything2 or H2G2

Rehber Ansiklopedisi - online version of the Turkish Encyclopedia "Yeni Reber Ansiklopedisi" (New Guide Encyclopedia), a common reference source

SourceWatch (formerly Disinfopedia) - a project to expose propaganda and paid spin doctors

Turkcebilgi - a popular online Turkish encyclopedia

Veropedia - mirrors stable versions of selected English Wikipedia articles.

Wikipedia - the largest encyclopedia in the world with over 2,000,000 articles in the English version alone (including minor articles and stubs)

WikiPilipinas - a take on the Wikipedia model from the Philippine point-of-view

Several online encyclopedias centre on mathematics:

MathWorld - a proprietary system hosted at Wolfram Research.

PlanetMath - a free Wiki-style mathematical encyclopedia which was originally built to replace MathWorld, a proprietary system hosted at Wolfram Research which was down for some time due to legal difficulties. Since MathWorld has returned, PlanetMath has still thrived.

The QED Project - quoted as a "distributed, computerized repository that rigorously represents all important, established mathematical knowledge."