 Encyclopedias
Sometimes you just need a bit more information.
Know Play 
It provides a neat front end to sources for answering quick reference enquiries. UK readers will find great benefit from using the general and reference sources.
xrefer
A single interface enables you to search a huge range of encyclopedias, dictionaries and quotations.
Bartleby.com
A library on a single site, Bartleby provides a dictionary, encyclopedia, thesaurus and much more.
The Britannica Concise
Encyclopedia Britannica is no longer be free. If you need encyclopedia access but don't want to pay for a subscription, you might want to take a look at Britannica Concise. It's not as thorough as the full version, but it's free. You can search by keyword, browse by letter, or check out the entry and illustration indexes. Worth a look.
Encarta Online
Quite a lot of information freely available.
The 1911 Edition Encyclopedia Britannica
"The best encyclopedia ever written was published nearly 90 years ago! And now you can find right here on the web! This 1911 edition of the Encyclopedia Britannica is filled with historical information that is still relevant today."
How stuff works
A very useful site which does what it says.
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