Encyclopedias

How Stuff Works - Articles explain how hundreds of things work, from elements of the human body to space-age technology.
http://www.howstuffworks.com/

Classic Encyclopedia - Browsable online version of the 1911 edition of the Encyclopaedia Britannica.
http://www.1911encyclopedia.org/

HighBeam Encyclopedia - Offers articles from the Columbia Encyclopedia and Britannica Concise Encyclopedia.
http://www.encyclopedia.com/

World Book Encyclopedia - Online version of the complete reference work along with dictionary, atlas, links, magazines, historical documents, audio, video, images, and 3D photographs. Requires free temporary registration with option to buy continued access for full articles.
http://www.worldbook.com/

Online Encyclopedia - Offers articles from on the 11th edition Encyclopedia Britannica, and from the Crystal Reference Encyclopedia. All articles are searchable and alphabetized.
http://encyclopedia.jrank.org/

Simple English Wikipedia - User-contributed online encyclopedia intended for people whose first language is not English.
http://simple.wikipedia.org/

Information Please - Online dictionary, Columbia Encyclopedia, and almanac reference.
http://www.infoplease.com/

Encyclopaedia Britannica Online - Provides complete text of Encyclopaedia Britannica with search capabilities, related links, and multimedia enhancements. Full text for subscribers only; free trial available.
http://www.britannica.com/

Knol - Peer-reviewed encyclopedia from Google. Articles are written by named contributors.
http://knol.google.com/

Encyclopedia Smithsonian - Links to on-line Smithsonian resources and answers to frequently asked questions from A to Z.
http://www.si.edu/Encyclopedia_SI/

Bartleby.com - Includes the Columbia Encyclopedia, Sixth Edition, dictionaries, and thesauri.
http://www.bartleby.com/

Wikinfo - User edited encyclopedia based on Wiki software. Contains articles, information on editing, and a list of differences between this encyclopedia and Wikipedia.
http://www.wikinfo.org/

Hutchinson Encyclopedia - "A subset of the content (concise set) is provided for free, the access to the rest of the Hutchinson Encyclopedia requires subscription."
http://encyclopedia.farlex.com/

Britannica Concise - Featuring 28,000 short articles.
http://concise.britannica.com/

Nuttall Encyclopædia of General Knowledge - Approximately 14,000 entries on topics including art, geography, literature, philosophy, religion. Originally published in 1907.
http://www.fromoldbooks.org/Wood-NuttallEncyclopaedia/

Encyberpedia - Electronic encyclopedia with its own content as well as over 10,000 links to the best reference sites on the Internet.
http://www.encyberpedia.com/cyberlinks/links/index.html

The Probert Encyclopaedia - Reference source with entries covering a broad variety of subjects.
http://www.probertencyclopaedia.com

Wikipedia - A free encyclopedia with millions of articles contributed collaboratively using Wiki software, in dozens of languages.
http://wikipedia.org/