NEW!!    http://projecteuler.net/
Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

http://www.nlvm.usu.edu/en/nav/vlibrary.html
Manipulatives and concept tutorials for mathematics instruction (K -12) emphasis.

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Explore a world of math wehre difficult problems can be fun and discover hundreds of resources, exercises, and an amusement park devoted exclusively to math.