Physics interactive simulations

I wanted to share an exciting new product launch on CK-12. They have taken the next step in digital learning by building high quality free Physics interactive simulations - all free! These simulations encourage students to explore, engage and learn by blending the physics, abstract and real world experiences around them. Check them out here:http://interactives.ck12.org/simulations/
You can also learn more about the free, editable textbooks that CK-12 offers by looking at this post.

What types of simulations do you use?