It’s really superfluous to add this link, because everyone with an interest in Maths will know of it. How lucky we are to have this wonderful resource!
The Wikipedia is also a very extensive maths encyclopedia. It is worth reading both the Mathworld and the Wikipedia articles on a particlar topic of interest. Every Wikipedia article I have read cross-references the relevant Mathworld article.
The macTutor History of Mathematics archive
The MacTutor History of Mathematics archive has at this link a very extensive encyclopedia of biographies of mathematicians.