For my technology and mathematics education class, we compiled a list of useful math resources. I was in charge of the section for younger kids and mathematics (you'll have to scroll down to find my section). I assembled links to sites with great resources for helping your child (even infants and toddlers) learn mathematics. It's never too young to start. And if you aren't too keen on mathematics, most of the sites have information about other subjects as well.
I strongly recommend the PBS site (thanks Kristi) and the Department of Education site.
And for those of you who just check this blog to see pictures of Grant Quayle, here's a picture of Grant after doing some fun math!