Encyclopedias

Citizendium
Wiki with stricter editing rules and obligatory disclosure of editor's real names.
http://en.citizendium.org

Connected: An Internet Encyclopedia
The engineering, theoretical concepts, and organizations of the Internet (freely distributable).
http://www.freesoft.org/CIE/

Cunnan
An encyclopedia for reenactors of the Middle Ages and Renaissance period with a heavy slant towards members of the Society for Creative Anachronism.
http://cunnan.sca.org.au

dKosopedia
A collaborative project of the DailyKos community to build a political encyclopedia.
http://www.dkosopedia.com/

Everything2
An extensively hyperlinked collection of facts, ideas, notes and humor to which anyone can add.
http://www.everything2.com/

Memory Alpha
A peer edited Star Trek encyclopedia.
http://memory-alpha.org

Musipedia
A collaborative music encyclopedia. Contains melodies and musical themes from classical, popular, and folk music. Can be searched based on the melody alone, either by playing it on a keyboard, whistling or singing, or by entering the melodic contour as Parsons Code.
http://www.musipedia.org

Open Site
A volunteer-run open content encyclopedia.
http://open-site.org/

PlanetMath
Collaborative, peer-reviewed mathematics encyclopedia with TeX input, inspired by MathWorld (GNU Free Documentation License).
http://planetmath.org/

Plastics Wiki
Wiki-based website dedicated to plastics technology, polymer materials, equipment and other things related to polymers and plastics.
http://plastics.inwiki.org/

SourceWatch
A directory of people, organizations and issues shaping the public agenda. It catalogs PR firms, activist groups and government agencies as well as the criticisms that are made of these groups from different perspectives.
http://www.sourcewatch.org/

Technomanifestos Network
Collaborative resource on the history of computing and the Internet (GNU Free Documentation License), and the online companion to the book Technomanifestos.
http://www.technomanifestos.net

Wikipedia Article - Nupedia
Nupedia was a public peer-reviewed general encyclopedia created by volunteer scholars, with resources for readers and contributors (GNU Free Documentation License). It can be considered to be Wikipedia's direct ancestor.
http://en.wikipedia.org/wiki/Nupedia