Everything you could possibly need about math and much more...
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For Reference and General Math Help...

Rating Site Name Description
***** Mathworld A great resource with very specific and advanced topics.
***** Provides in-depth lessons and reviews for topics ranging from basic math to calculus and more. (Includes loads of proofs!!)
***** Mrs. Glosser's Math Goodies Interactive Lessons, puzzles, worksheets, and active forums for math discussions.
**** Automatic Calculus and Algebra Step-by-step solutions in a number of topics; great for checking homework or reviewing for exams.
**** St. Andrews Archive This is The MacTutor History of Mathematics archive, a good source for researching the history of mathematics.

Five College Mathematics Departments

Mount Holyoke
University of Massachusetts
Hampshire (Natural Science)

Math Organizations

American Mathematical Society
Mathematical Association of America
Association for Women in Mathematics (see also the AWM Mentor Network)

William Lowell Putnam Mathematical Competition

General Information
Problems & Solutions Archive

Fun Math Stuff...

Rating Site Name Description
***** Cut the Knot: Interactive Mathematics Miscellany and Puzzles Lots of fun facts and a huge collection of math-oriented puzzles
**** Games From, some nice brain-teasers with decent graphics.
**** Clever Games for Clever People Unique math games to try when you really want to strain your brain. (Develop strategies and win everytime)
**** Quia: Matching Graphs to Equations Refresh your memory of some of the basic graphs from Calc 1. Choose a matching game, memory game, or flashcards for review
** Pick a Number Game Simple game for at least 1 minute of entertainment...
Biolographies of women mathematicians
A site hosted by Agnes Scott College
National Curve Bank
All kinds of neat curves, many interactive, on a wide variety of topics.  Submitted by mathematicans nationwide. 

Non-Math Fun

(For when you have MATH overload... ;)

Rating Site Name Description
***** FreeWorldGroup Games Tons of free games to play online, download, or play in tournament mode (compete with others on the web).
***** eBaum's World Extensive media collection of all things funny!