The Wolfram Demonstrations Project produces some of the best interactive demonstrations of mathematics concepts that I’ve found on the web. The list of demonstrations is very comprehensive and offers something for every level of mathematics study. The demonstrations can be viewed online, but to make to use the demonstrations interactively you need to download the free Mathematica Player from the Wolfram website. The Mathematica Player is available for Windows, Mac, and Linux environments. Installing the Mathematica Player allows users to download demonstrations and use them interactively on their local computer. The demonstrations available range some very simple concepts like estimating and multiplication of simple numbers to Calculus and beyond.
Applications for Education
The demonstrations available from Wolfram are suitable for upper elementary school students through collegiate level mathematics students. The interactive demonstrations allow students to experiment with mathematical variables and see the effects of those variables. What is nice about many of the interactive demonstrations is that students can experiment with demonstrations then have the demonstration reveal the answer or rule that applies to the problem.
The image below is linked to an elementary school mathematics demonstration from Wolfram.