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Reference Material

Finding People

WhitePages.com Find telephone numbers, addresses, and full zip codes of people and businesses in the U.S.A.
Switchboard Find telephone numbers, addresses, and much more
Infobel World
Telephone Directories
Telephone directories worldwide.

Dictionaries and Thesauruses

dict.org Search many dictionaries all at once, including 1914 Webster's, modern, geographical, and others.
WWWebster The Merriam-Webster Dictionary on the Web.
Roget's Thesaurus on the Web Probably the most powerful thesaurus in the world.
travlang's Translating Dictionaries A great many dictionaries, from many languages to many other languages.
MedlinePlus Medical Dictionary An online medical dictionary at Medline (the U.S. National Institutes of Health).
Stedman's Medical Dictionary Stedman's standard medical dictionary, in online form.
Dorland's Medical Dictionary Dorland's standard medical dictionary, in online form.

Maps

Yahoo! Maps Online road maps plus driving directions.
MapQuest Online road maps plus driving directions, one of the first to do it. Very pasteable, unlike the others.
Google Maps Mapping & satellite imagery web site from Google. Very interesting, and often more close up than others.
National Geographic Map Machine An online atlas with lots and lots of good maps of regions and nations around the world.

Encyclopedic and General

Search Wikipedia at Answers.com Wikipedia is an excellent and exhaustive general reference.   Answers.com keeps a very speedy mirror.  The English home page is at en.wikipedia.org, but the Answers.com site is recommended for general use, for speed.
Information Please General reference online. Statistics, maps, dictionary, and encyclopedia.
Conversion & Calculation Center A great place to convert Centigrade to Fahrenheit, gallons to liters, bushels to drams, kilograms to grains, and thousands more. Also delivers currency exchange rates, world time zones, and other useful data. For units of electrical power (kV, kVA, kW, HP, I, etc.), try this link.
The Internet Public Library, Reference Center The IPL is at the University of Michigan School of Information. Their Reference Center has all of the basic reference tools, plus a lot.
CIA World Factbook The U.S. Central Intelligence Agency's Factbook, containing much data, especially concerning every nation in the world.

Miscellaneous

RhymeZone An excellent rhyming dictionary at Lycos.
Behind the Name Origin and history of first names.
Behind the Name, Surnames Origin and history of last names.