The sources below are on many topics. See individual fields of physics for other interactive sources.
Physics Web - interactive experiments. The best listing of physics applets, by Institute of Physics.
Physics 2000. Excellent interactive tutorials on interference and lasers.
By Fu-Kwun Hwang. These are the best applets! They are easy to use, and you continue to learn as you use it.
ExploreScience has excellent, simple, informative, interactive Shockwave experiments.
Physlets. Excellent Physics Applets designed for student learning, from Davidson College.
Fear of Physics is a collection of interactive experiments for the general public or students.
65 nice applets for mechanics, waves, etc. but in Spanish.
Italian applets. These are very nice, even if you don't read Italian.
Explanations and small versions of applets on web, full-size on commercial CD.
Jack's Page. Applets for first year college physics.
Fun@learning.physics. Free sample applets: projectile motion, elastic collisions, charged particle in magnetic field.
General Physics Lab, Mechanics. U. Kentucky. Applets in mechanics, plotting graphs, forces, projectile motion, sliding friction.
Virtual Laboratory. Mechanics, thermodynamics, astrophysics, and electricity by U. Oregon.
Applets on math, kinematics, waves, gravitation.
Interactive Matter. Has interactive units on matter, circuits, fusion, etc. By Princeton Plasma Lab
A forum for computer simulation methods.
The Physics Classroom. Tutorials, animated gifs, shockwave, questions done by students.