I think Freeze Ray is a very cool site. It has many interactive activities that demonstrate abstract concepts in a very concrete way. For example, when explaining the concept of using a convex or concave lens to adjust vision in glasses, it literally shows how the image changes with the lens in place using the slide bar at the bottom of the screen. That is under the physics category in the Eye Defects lesson.
There are many other valuable interactive activities as well. You should really check it out!