I’ve added a bit of a hardcore link today so brace yourselves!
The mathematical atlas is a site that contains information on all the modern areas of mathematics that are studied in dark, tea/coffee fuelled, tweed, cordoroy and elbow-pad infested research institutions around the world. There is a guide for newcomers at the bottom of the page that will give you a nice round introduction to our world, and for the more brave amongst you, the rainbow links on the right of the page go straight in for the jugular! I would say I was a 20, 05 with a handful of 68 (if you’re interested).
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That is a great link. But it is a little hardcore. You should mention Mathworld (http://mathworld.wolfram.com/) as a better starting place for most subjects.
Another hardcore maths link worthy of a mention is John Baez’s site in all its mathematical physics-y goodness! http://math.ucr.edu/home/baez/README.html