General Math
National Library of Virtual Manipulatives
Open Directory for Math and Science Applets
Math Playground - Math games K-8
A+ Math - Great resource for basic math. Includes games, puzzles, and interactive flash cards
Fun Mathematics Lessons - Good General Math resources
Problems with a Point - Find problems you can use in your classroom.
NCTM Electronic Examples - Activities that Focus on the National Standards
Interactive Mathematics - Learn math by playing with it.
Figure This - Math challenges for families
VisualFractions - I think it does a nice job of doing just that