From Yves Pelletier:
YP Waves simulates up to 3 traveling sine waves simultaneously. It is easy to change the amplitude, wavelength, velocity and phase constant of each wave. This program is perfect to teach waves characteristics (amplitude, wavelength, etc.), the superposition principle, standing waves, interference of waves, beats, etc.
YP Waves runs on any Macintosh computer with MAC OS 7.0 or later. It needs about 200 Kb on your hard disk and 800 Kb of RAM. The software runs on older models with 68000 processors (Mac Plus, Mac Classic), but simulations are slower.