Here are some links to a few mathematics resources on the Internet that I have found useful or interesting. I long ago gave up any effort to construct a comprehensive list.
Become an Expert! - A Guide to Fraction Fun
This site was recommended by Beth Mowry and her Mathematics Students in Northern California
Fraction Fun for Kids
This site was recommened by Samantha, a student in Ms. Smith's 4th grade class at Brighter Futures Charter School in California.
Dave's Math Tables
Lots of formulas, identities, tables, series, etc.
The Fractal Microscope
Furman University Mathematical Quotation Server
The Gallery of Interactive Geometry
Graphics for the Calculus Classroom
The Mandelbrot Explorer
Online Mathematics Dictionary
This Interactive Online Mathematics Dictionary was developed for use by middle school mathematics students as well as the parents and teachers of middle school mathematics students. The language of the definitions attempts to be consistent with middle school levels of rigor. The examples, links, tests help to elaborate on the definitions. Although developed as a separate project, it was incorporated into the InterMath Project and has been adopted by the Georgia Department of Education.
Project InterMath was funded by the National Science Foundation as a collaborative effort of the Department of Mathematics Education and the Learning and Performance Support Laboratory (LPSL) and the CEISMC group at the Georgia Institute of Technology. InterMath is a professional development effort designed to support teachers in becoming better mathematics educators. It focuses on building teachers' mathematical content knowledge through mathematical investigations that are supported by technology. InterMath includes a workshop component and materials to support instructors.
Visual Dictionary of Special Curves
MacTutor History of Mathematics Site -- St. Andrew's University
Excellent site for History of Mathematics. Also, interactive Java graphs of many curves.
Jim's GSP Library
Some Sample GSP Lessons