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Ask Dr. Math - Students from kindergarten through high school can scan math questions to see if their own have already been answered, or they can ask Dr. Math, which is actually a group of math whizzes from colleges around the country.
Encyclopedias - These encyclopedias are free of charge.
http://www.britannica.com - Encyclopedia Britannica
http://en.wikipedia.org - Wikipedia
Carnegie Library of Pittsburgh: Kid's Home - Online help from real tutors or search their database.
Homework Help - This site has over 625 links on subjects that range from art to the Internet. It is run by a thirteen year old middle school student and his father. Together they answer over forty questions a day from students around the world. http://school.discoveryeducation.com/homeworkhelp/homework_help_home.html
Homework Spot - Sections for elementary, middle school and high school. Has a current events section and a section to visit museums and libraries.
Homework/Study Tips - This site offers a comprehensive list of annotated links which are best suited for older elementary school students or middle school kids.
Khan Academy - A library of over 3,000 videos covering everything from arithmetic to physics, fincance, and history and 317 practic exercises. There are videos for all grade levels.
SOS Math - This site provides help with math homework.
Ask-A-Geologist - Send your question relating to geology.