This site features three simple-to-use applets: contour diagrams, curve families, and surface of revolution. The applets are written in Java and exploit Sun Microsystems' Java OpenGL technology so they will run on Apple OS X, Microsoft Windows 2000 or above, Solaris and many common configurations of Linux.
The "area function" is often discussed as part of an introduction to the definite integral. This applet provides an innovative approach to producing functions drawn with a mouse in the context of an exploration/demonstration of the area function.
This Flash Forum developer's article includes source code and instructions for building an applet that allows the user to enter expressions with up to four parameters and then to vary the parameters in order to fit the curve to user data.