In 1993, a project called Interpedia was being discussed; 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 and the emergence of high-quality search engines.
The most profound output of these might be the early proposals, especially from the Millennium Project of the United Nations University in 1993, to filter the material for religious and ethical alignments that might make it easier to manage the divergence of values in any global project.
Anticipating major problems too early may have been one of the reasons these projects did not succeed. Converting existing material was less controversial.
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.
Everything2 has a wider range and does not exclusively focus on building an encyclopedia; its contents are not available under a copyleft license.
Indymedia, which focuses on networking first-hand source material from local, diverse groups of people around the world, linking grassroots, non-virtual social reality and the internet community.
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.
EvoWiki is "a Wiki about evolution and origins. The focus is on evolution education, particularly addressing the arguments of Creationism and Intelligent Design from the perspective of mainstream science. It is inspired by webpages such as talkorigins.org and talkdesign.org, and the goal of EvoWiki is to complement rather than duplicate these online resources".
PlanetMath is 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.
Gnupedia, an initiative which didn't come to fruition but had some interesting philosophy.