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Math

AAA Math
http://www.aaamath.com/
Explanations, practice problems, and challenge games for every level from simple addition to algebra and geometry.

Ask Dr. Math
http://mathforum.org/library/drmath/drmath.high.html
Math problems from algebra to trigonometry.

FunBrain!
http://www.funbrain.com/
Math games for all ages!

Math.com Homework Help
http://www.math.com/students/homework.html
Homework help for pre-algebra, algebra and geometry.

Math League Help
http://www.mathleague.com/help/help.htm
Problem solving tips and graphics to help you with decimals, fractions, geometry, and more.

Trigonometry
http://aleph0.clarku.edu/~djoyce/java/trig/
A quick course on trigonometric functions.

Webmath
http://www.webmath.com/
Homework help and word problems in math topics for grades 7-12.