Plus Magazine is an online magazine about mathematics.
Art of Problem Solving is an online community of math teachers and students of all levels from elementary school through graduate school.
Wikipedia is an online encyclopedia with extensive coverage of mathematics, including detailed descriptions of mathematical terms and theorems. There is also a lot of information of prominent mathematicians, past and present.
PlanetMath has a mathematical encyclopedia as well as free online expository papers and books
MathWorld is a online math reference maintained by Wolfram research.
Textbooks in Mathematics contains an extensive list of links to free math textbooks available on the Internet.
Society for Industrial and Applied Mathematics is the leading professional society for applied mathematicians working in either academia or industry.