Sumanas has some great, free animations for science classes. As you can see from the home page shot above, the categories are wide ranging and include Biology, Chemistry, and even some math. I have used many of them and found them to be very good and nicely designed animations that explain topics well. Some of the topics are pretty advanced and may only be relevant to AP or college courses.
This can be used by the teacher in the classroom or students can be directed here on their own as a study resource.