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.
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.
Math Overflow is a question and answer site for professional mathematicians.
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.
Society for Industrial and Applied Mathematics is the leading professional society for applied mathematicians working in either academia or industry.