The Physics Education Technology (PhET) website from the University of Colorado on Interactive Simulations
Here is an example of an elementary simulation on fractions.
Here is another one for middle school on Plate Tectonics.
Not to leave out a high school example, here is one on graphing equations.
The BioDigital Human is a website dedicated to a 3-D model of the human body. I know that it is very specific to science, but I though it worth throwing out there to balance things out a bit and challenge those language teachers among us to come up with a creative idea for it's use in class.
If you are a math or science teacher, I challenge you to use at least one of the simulations in class and report about how you used it and how it went. If you are not a math or science teacher... I challenge you to share a way that you can use these or other interactive simulations in your classrooms.