by S.Kamikawa, high school Physics teacher in Japan.

http://www.bekkoame.or.jp/~kamikawa/java_e.htm/

A variety of simulations for Internet Explorer 3.0. These are simple but show the patterns of waves nicely:
Longitudinal Wave; Reflection of Longitudinal Wave from a Boundary;
Transverse wave and Longitudinal wave; Simulation of Transverse wave and Longitudinal wave;
Wave Interference, Animation of Interference Pattern Formed by Two Points Sources.

Fu-Kwun Hwang, National Taiwan University Department of Physics.

http://www.phy.ntnu.edu.tw/~hwang/

A nice series of wave and oscillation applets:
Superposition Principle of Waves
Fourier Synthesis
Transmission of Waves (Reflection and Refraction)
Interference Between Two Waves (Two Point Sources)