An encyclopedia of over 2500 symbols. Search by keywords/concepts or by graphical attributes.

The Probert Encyclopaedia is an extensive free online encyclopedia and dictionary of over 300,000 cross-referenced and interlinked topics with over 16,000 photos and illustrations and over 2000 maps, specialising in information not found in other encyclopedias and dictionaries.

Peer-reviewed encyclopedia from Google. Articles are written by named contributors.

Wikipedia is a free, open content, community-built encyclopedia with thousands of articles on topics from A to Z. Available in dozens of languages.

Browsable online version of the 1911 edition of the Encyclopaedia Britannica.

Bartleby.com publishes thousands of free online classics of reference, literature and nonfiction

Reference.com - a free online encyclopedia and information reference. Research articles from Columbia Encyclopedia and Encyclopedia Britannica.