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Online version of the complete reference work along with dictionary, atlas, links, magazines, historical documents, audio, video, images, and 3D photographs.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/The Probert Encyclopaedia
Reference source with several hypertext linked entries covering a broad variety of subjects.
http://www.probert-encyclopaedia.co.uk/
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 LoveToKnow Free Online Encyclopedia
Browsable. Based on the eleventh edition of the Encyclopaedia Britannica, first published in 1911.
http://www.1911encyclopedia.org/
How Stuff Works
Articles explain how hundreds of things work, from elements of the human body to space-age technology.
http://www.howstuffworks.com/
HighBeam Encyclopedia
Offers articles from the Columbia Encyclopedia and Britannica Concise Encyclopedia.
http://www.encyclopedia.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/
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/
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/
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/
Britannica Concise
Featuring 28,000 short articles.
http://concise.britannica.com/
Bartleby.com
Includes the Columbia Encyclopedia, Sixth Edition, dictionaries, and thesauri.
http://www.bartleby.com/
MSN Encarta Encyclopedia
Includes a world atlas, dictionary, thesaurus, multimedia collection, and homework tools.
http://encarta.msn.com/
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/
Wikipedia
A free encyclopedia built collaboratively using Wiki software. (GNU Free Documentation License) and with over 1,000,000 articles. Note that because articles can be "vandalized," information on Wikipedia is not always accurate.
http://en.wikipedia.org/wiki/Main_Page
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/
Encyclopedia Smithsonian
Links to on-line Smithsonian resources and answers to frequently asked questions from A to Z.
http://www.si.edu/Encyclopedia_SI/
