Bartleby’s Reference Guides
This is the ultimate of reference sites, with full text online copies of a huge list of dictionaries, encyclopedias, almanacs, style guides, etc. You can also search other full-text resources in literature, including poetry (verse) and nonfiction by clicking on the tabs at the top of the page.

Credo Reference
xrefer contains encyclopedias, dictionaries, thesauri & books of quotations from the world's leading publishers. All cross-referenced, all in one place - providing you with a single source for reliable factual information.

Conversion Calculator 
A great online resource for converting measurements & weights.

RefDesk
This is a great big site with loads of reference materials, including news, facts and encyclopedias. This is a great general site for getting the best reference material on the web.

CIA Fact Book
Read what the spies read before they go on a mission to a foreign country. This site will give you lots of detailed facts about foreign countries. It really is from the CIA!

Language Tools
At this site you can look up words at the online dictionary, check a thesaurus or even translate a web page into whatever language you want!

Style Guide - Elements of Style
This is the definitive style guide for usage of the English language in your writing.