PhET Sound & Waves Simulations
This link takes you directly to the PhET collection of 9 simulations on sound and waves. It includes introductory concepts of wave properties and interference, plus "Normal Modes", "Optical Tweezers", "Microwaves", and a PhET favorite: "Fourier - Making Waves".

Physlet Physics 3E: Waves
This newest edition of Physlet-Waves contains 24 models on Periodic Motion, 30 models on Waves & Oscillations, and 26 on Sound. Each model focuses on one essential concept, with added complexity in the "Problems" sections. Many of the models have accompanying pdf worksheets.
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PhET Light & Radiation Collection
This is a direct link to PhET's 24 simulations relating to light, optics, and radiation. It includes six HTML sims for beginners that open easily on mobile devices, plus Java simulations relating to MRI, quantum wave interference, lasers, blackbody radiation, Greenhouse Effect, and more.

Waves: An Interactive Tutorial
Here's a robust collection of 33 device-friendly simulations with study questions, created by veteran physics professors Kyle Forinash and Wolfgang Christian. The sims are focused on properties & behaviors of waves that are hard to understand by looking at a textbook page.

Physics Classroom: Waves and Sound Interactives
These six simulation-based tutorials are very easy to use and all come with text-based student activities or exercises. Topics include masses on springs, Slinky lab, simple mechanical waves, wave addition, standing wave maker, and beats.

Physics Classroom: Light and Color Interactives
This page has a collection of 5 simulation-based tutorials on color and one sim on Young's Experiment. The interactives on color are among the few free simulations on this topic which include related text-based activities and exercises suitable for introductory physics.

Physics Classroom: Refraction and Lenses Interactives
Here are 5 simulation-based tutorials on lenses and refraction, each providing a related student activity and/or exercise. Included are a virtual optics bench, image formation in converging and diverging lenses, a sim on Least Time Principle, and a refraction interactive that lets students choose from among different media and modify the angle of incidence.

Boston University: HTML5 Physics Simulations
This link takes you to the full collection of Andrew Duffy's HTML5 physics simulations. NOTE: You must scroll halfway down the page to find the 15 "Waves" sims, which cover fundamental wave properties, wave speed, Doppler Effect, and interference concepts.
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Wave Representations Model
This Open Source Physics model displays motion of a wave on a string alongside a Position vs. Time graph for two points on the string. Using only these representations, you can determine amplitude, period, wavelength, and wave speed. Set the parameters for your wave. Can you correctly answer the auto-generated questions? The model is written in Easy Java Simulation -- opens quickly and runs well on most devices except phones.

Academo Physics Simulations: Light
Academo is an online hub of free sims and visualization tools built by an active open-source community. The link below takes you directly to five Academo physics resources on light. All open easily on PC, Mac, and mobile.

Academo Physics: Waves Simulations
Academo is an online hub of free sims and visualization tools built by an active open-source community. The link below takes you directly to four Academo physics resources on waves, including 1D Wave Equation, Amplitude Modulation, Spherical Waves, and a Wave Interference & Beat Frequency generator. All open easily on PC, Mac, and mobile.