AAAmath.com's pages illustrate and provide interactive arithmetic exercises and problems. They contain a series of basic math lessons which are a resource that can be used by math students in formal elementary education math classes, home schooling or elsewhere.
Cut the Knot.org has interactive mathematic Miscellany and Puzzles for all ages and skill levels.
Math2.org has math help for all ages and skill levels.
PlaneMath.com is a place to learn cool things about math and aeronautics!
The Geometry Center
The Geometry Center is dedicated to help kids with their geometry questions and/or problems.
The Math Archives
Math Archives has a long list of Internet sites that can be used when teaching mathematics at the K-12 level.