Math League: math contests, grades 4-12
MathFax: math contests, grades 3-12
Math.com: the math world online
Metric Conversion Tools: online calculators for metric conversion
Math Fact Cafe: Create worksheets, print site-created worksheets
Virtual Manipulatives from McGraw Hill
Calc101.com: step by step derivatives and integrals, each step explained; long multiplication, division of polynomials
Dataplot: statistical software, especially for linear and multilinear regression, nonlinear modeling, related diagnostics; from National Institue of Standards and Technology
Fractint: generates pretty fractals, deep zooming

Do you know of other online resources for math? Leave a comment below.

Online Resources for Math