The CRC Encyclopedia of Mathematics - Huge encyclopedia with nearly all Math terms explained. Bad news: Entries beginning with 10 randomly chosen letters are blocked each day.
Frequently Asked Questions in Mathematics - Before posting a question in the sci.math newsgroup, you should first look for the answer in this FAQ. (It's hardly comprehensive, unsurprisingly, as math is such a vast subject.)
Knot a Braid of Links - Your rough guide to mathematics on the web. A new site is added each week. Unsorted.
Mathematics by Classifications - Categorised directory of links to journals, preprints and abstracts, from the American Mathematical Society - organized according to the standard Mathematical Subject Classification.