The most popular sites in all 'Reference' categories, according to the Alexa Toolbar. Updated daily.
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1.
An online collaborative encyclopedia.
wikipedia.org
Site info for wikipedia.org 
2.
Provides directions, interactive maps, and satellite/aerial imagery of the United States. Can also search by keyword such as type of business.
maps.google.com
Site info for google.com 
3.
Online version of Roget's II: The New Thesaurus, 3rd edition.
thesaurus.reference.com
Site info for reference.com 
4.
Find directions for and explore towns and cities worldwide. Display addresses on a map, view nearby businesses, get driving directions and maps, and plan a trip with city information. Also includes aerial photographs of selected areas.
www.mapquest.com
Site info for mapquest.com 
5.
Allows two-way searches for over 400,000 words and phrases. Several options available.
dict.leo.org
Site info for leo.org 
See top sites in all 'Reference' categories