Figure This: Math Challenges

http://www.figurethis.org/index.html

A website for families, teachers, and kids to work on math challenges in a fun learning environment.  The website is supported by the National Science Foundation and the US Department of Education. 

Smarty Games

http://www.smartygames.com/math.php

SmartyGames is a free educational website for kids. It features fun math games, mazes, puzzles, coloring pages, animated flashcards and online children's books.

Math is Fun

http://www.mathsisfun.com/

A lively site with information and activities on many different areas of math.

Great Websites for Kids - Mathematics

http://www.ala.org/gwstemplate.cfm?section=greatwebsites&template=/cfapps/gws/displaysection.cfm&sec=26

A wonderful list of math sites for children from preschoolers to middle school students. Something for everyone.