General Reference Portals
refdesk - Great place to find all types of reference materials ,and other information.
InfoPlease - Excellent site for references especially encyclopedias.
Library Spot - very good site for references and other resources such as top books.
Merriam-webster - Very good site that includes thesauras and Spanish/English.
YourDictionary - Very easy to use with lots of resources.
Dictionary.com - Very simple and easy to use.
Encyclopedia.com - great site and very easy to use.
Wolfram Mathworld - Excellent Math Encyclopedia.
Eh Net - Economic and Business encyclopedia.
Science Encyclopedia - very good science encyclopedia.
CIA World Factbook - most up to date and complete almanac.
50 States - Good for studying the U.S. states.
World Atlas - Good for studying the world.
National Atlas - Very specific maps available on the site.
University of Texas Map Collection - Most complete online map collection available.
Conversion & Special
Online Conversion - Convert all kinds of measurements.
XE - Currency Converter
Quoteland - Quotes on every major subject.
On This Day - Find out what major events happend on each day of the year.