If you're going after students, have a look at http://www.mathletics.ca/ http://www.mathletics.ca/ and http://www.rainforestmaths.com/ http://www.rainforestmath

If you're going after students, have a look at http://www.mathletics.ca/ http://www.mathletics.ca/ and http://www.rainforestmaths.com/ http://www.rainforestmaths.com/ We signed up our kids for a year for ~$200. It was pretty good, but there are ways it could be improved, such as not using Flash everywhere. But there's a lot of activity on that site, and therefore a lot of money to be made.