I wanted to put my two cents worth in - I have a degree in the Secondary Education of Mathematics from Penn State University. Depending on your general knowledge of mathematics already, I have to say - to learn what you would call "Advanced" mathematics - meaning number theorey - non-euclidean Geometry, Game Theorey and so on - you would really need to invest some money and take some classes. The books that are out there on these topics assume that you know too much already. In all "Advanced" mathematics I took classes in, we had 2-3 weeks of learning the basics of a subject before we even got to where teh books began because of their assumptions of basic knowledge on the subject. To my knowledge, the literature on online availability of some of the Advanced Mathemtatics is one area that is really lacking.

Sorry to be the bearer of bad news - but truly Advanced Mathematics (beyond Calculus) - really would be difficult to learn from a book or from online -

That's just my opinion - maybe someone else on here can enlighten me on some good resources, Lord knows I've tried to find them myself.