Wolfram Research
Wolfram Math World: World's Most Extensive Mathematics Resource
MathWorld provides a huge mathematics encyclopedia detailing the concepts of geometry, algebra, calculus, discrete math, number theory, and other math topics--complete with formulas, diagrams, examples, and references for thorough...
Other
Allmath: Math Glossary
Are your students having difficulty understanding math terminology? Try this A-Z glossary on math related terms. It's quick, easy to read, and very complete.
University of Vienna (Austria)
University of Vienna: Maths Online
The University of Vienna, Austria, created this site to help bring the worlds of math and technology together. You have access to many applets and downloads that allow you to create lessons, tests, and puzzles. There are also great...