The game of Set. If you haven't tried this one yet, don't wait any longer. http://www.setgame.com/
Check out the "Explorer" section. A nice math site. http://www.counton.org
Lots of math games. http://www.funbrain.com/math
Colors and text are sometimes overwhelming, but it's a rich math site. http://www.coolmath4kids.com/
Look for the challenging puzzles on the bottom left...I love puzzles! http://www.mathsnet.net
Math flash cards, games, etc. http://www.aplusmath.com
Math facts and worksheets. http://www.mathfactcafe.com
Games, activities; find out how many seconds old you are! Strange cat, but cool web site. http://www.mathcats.com
Weekly brain teasers. I love brain teasers! http://www.eduplace.com/math/brain/index.html
Try some real-life problem solving with the entire family. http://www.figurethis.org
This is an addictive, fun numbers game! Who needs to buy books when this site is free with printable puzzles?! http://www.websudoku.com
Curriculum links for fourth grade math from the Wellesley math department. Keep this one bookmarked. http://www.wellesley.mec.edu/wes/pages/curriculum/4curr/4Math.htm?WebPageNo=18103
A terrific overall math site with challenges, learning experiences, and games. http://www.mathsisfun.com