The Function Institute from Zona Land

This gives comprehensive discussion on definition of a function with Java Applets that illustrates very clearly one-to-one, one-to-many, many-to-one concepts. It has pages on different types of functions (linear, polynomial, rational, exponential, trigonometric) and their interactive graphs.

http://id.mind.net/~zona/mmts/functionInstitute/functionInstitute.html

Functions, Graphing, and Modeling

A large interactive tutorial using a Java program as a graphing tool, aimed to prepare students for calculus.  Topics include functions, graphing, exponential, logarithmic, trigonometric, and algebraic functions.

http://campus.northpark.edu/math/PreCalculus/TOC.html

Maths Online Gallery: Functions 1

Collection of Java applets with exercises and some lesson plans: Function and graph, Recognize function graphs puzzles, Polynomial of third order, and a Function plotter.

http://www.univie.ac.at/future.media/moe/galerie/fun1/fun1.html#funktion

Maths Online Gallery: Functions 2

Collection of Java applets with exercises:  Recognize function graphs puzzles that concentrate on negative power functions and trigonometric functions, and graph collections of exponential & logarithmic functions, and trigonometric functions.

http://www.univie.ac.at/future.media/moe/galerie/fun2/fun2.html#graphenel