I've been having a look at the Teach Maths site and was intrigued by the GeoGebra animations they have made. I've been trying to work out how to make them but in the mean time here is an example of one that someone else made earlier. I'm afraid it's a bit too wide for the page so you can't use the controls but you get the idea.

The great thing about these is that they run on the webpage without having to go to another site so if you write a worksheet to go with it students can use the animation as they look at the questions. So far I haven't been able to do much more than make a circle but it's early days.