Through Robert Israel’s sci.math posting I’ve found an excellent online resource, especially for many statistical topics: Encyclopaedia of Mathematics. It seems better than the other two better-known options Wolfram MathWorld and Wikipedia. Another valuable resource Quantlets has several interesting books and tutorials, especially on the more financially oriented topics; while some materials are restricted, much of it is easily accessible. Finally, I have been impressed by Computer-Assisted Statistics Teaching – while it is of introductory nature, the nifty Java applets make it worth registering.
Encyclopedias, statistical resources, references