The site that you have recomended may be excellent MG but it also costs quite a lot of money :(
I do not know which free or cheap site is best.
If you know what you want to learn in a general sence then there are some really good sites in
mathisfun,
purplemaths,
Khan acadamy
I was just browsing and I stumled on this site - it looks like it might be good
http://interactivesites.weebly.com/math.html
ALSO
I just googled 'free math sites' and a whole lot of possibilities were thrown up.
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OH Here are some sites that MG just reminded me about (thanks MG) - I had put them in a previous post.
This site looks good and it appears to be free. If you join up they track your progress :)
http://www.mathopolis.com/list/index.php
Here is another site: