A+ Math - A really neat site with flashcards, game room, homework helper, and advanced problems. My kids think this is a "cool" site!
Blue Dog Can Do Math - Give the blue dog a math problem and he barks out the answer - so cute for the younger kids. When my daughter was in K, she just loved it!
Coolmath4kids - An amusement park of math and more designed especially for FUN!!!
Dave's Math Tables: Multiplication Table - Multiplication table to 12's. Also links to a few math sites, including games.
The Geometry Center - Contains current projects, multimedia documents, geometry reference archive, downloadable software, etc.
Math Archives - Contains lesson plans, software, resouces, and web-sites for K-12 math education.
Math.com - A world of math online! Everything from basic math to Calculus to Advanced topics. This is a great site!
The Math Forum - So much math information, so little time! You will find math resources by subject, discussions, problems of the week, web units and lessons, etc.
Math Forum Grades 3-5 - Conatins puzzles, problems, lesson plans, links, math software, etc.
Math Forum K-2 - Contains activities, projects, lesson plans, fun sites, etc.
Math Net Links - This is a collection of mathematical resources and related links.
Math Reference Materials - Part of the Math Forum Internet Resource Collection. Site contains so much information for all ages.
Math Reference Tables - From general math, to algebra, to geometry, to calculus - all the infor you'd need to find!
The Mathman - You will find sample worksheets from the Mathman's books, puzzles, games and hands-on activities, favorite places, etc. This is a site to sell the book, but it has a lot of free materials.
Math-U-See Homepage - Math-U-See is referred to many times on the HS lists I'm on, so I'm including the link here. I personally don't use this, so I cannot say if it is good or not.
MegaMath - An excellent site containing activities and lessons for elementary students.
Michele's Math - Contains all types of hints and helps to teach your child multiplication (using addition).
Middle School Math - Contains lesson plans and games!