National Library of Virtual Manipulatives
A library of interactive, web-based virtual manipulatives and concept tutorials, mostly in the form of Java applets, for K-12 mathematics instruction.

Visual Calculus
Demonstrations and explorations of many fundamental Calculus concepts, including limits, linear approximation and Riemann Sums.

The Wolfram Demonstrations Project 
An open-code resource that uses dynamic computation to illuminate concepts in science, technology, mathematics, art and finance.

Seeing Math™ Interactive Software
Interactive software tools to clarify key mathematical ideas in middle and high school mathematics.

Michael Buescher's Computer Resources for Secondary Mathematics
Worksheets for The Geometer's Sketchpad and activities on topics including sequences, Geomety, Trigonometry, exponents and quadratics.