The helper application approach has disadvantages and advantages. The main disadvantage is that the learner must have a copy of the software being used on his own computer or local network. The main advantage is that the learner does indeed have a copy of this software to use, with a full complement of features to experiment and work with, rather than a limited-feature applet or something else that won't allow him to save his work, for example. And serving helper application files requires merely configuring the correct server mime types; no special interface between remote software and the user is necessary.

The helper application approach is becoming more and more popular, especially for calculus: see for example the Calculus and Mathematica programme. (Mathematica also has a free MathReader that can be configured as a helper application).

A sample of other sites:

Vectors , Algebra and Calculus Sketches (GSP)

Introduction to Math and Spreadsheets (ClarisWorks)

Ordinary Differential Equations , Calculus (Maple)

various math and probability (Excel)

Mathview notebooks (MathView)

Calculus, D.E.'s, linear algebra, etc. (MathCAD, Maple, Mathematica, Matlab)