We've used physlets for a number of years to teach concepts related to electric forces, fields, and potentials. Simulations like these are an excellent way to cope with the fact that the experiments we could do for this collection of material don't deal with individual charges, and you can't see fields or potentials directly.
I'm not, however, enthusiastic about the fact that they're written in Java. When physlets originated, it was the most cross-platform, web-delivered solution, but technology has moved quite a bit since then. I wanted to see if I could duplicate some (or perhaps eventually all) of the functionality of physlets for electricity and magnetism using