The xFunctions applet covers several aspects of calculus and pre-calculus mathematics, including graphs, parametric curves, derivatives, Riemann sums, and integral curves.
Examine and tweak Mandelbrot Set fractal images. [Displaying the Mandelbrot set requires a great deal of computation. The user is best off with a fast machine, drawing small pictures.]
Mathematical Java applets with accompanying theory and tutorial sheets aimed mainly at undergraduate engineering students. Some applets also useful for K-12.