Marc Renault, supported by Shippensburg University made a complete library of applets for Calculus I that are suitable for in-class demonstrations and/or student exploration. He created these applets using GeoGebra software for creating dynamic visualizations of mathematical ideas.
My favorite applet I have used successfully in the classroom was the chain rule applet. It is a dynamic way to demonstrate the chain rule using an x-wheel, a u-wheel, and a y-wheel. You can change the speed of the x-wheel, and you can connect the wheels with belts and change their radii. This model is a great exploration of the chain rule and enhances an intuitive understanding of where the formula comes from. The best part is crossing the belts in the applet!
Marc Renault has a library of calculus applets created with Geogebra on his website:
Other Calculus Applets
This applet demonstrates the epsilon-delta definition of the limit. (In particular, we are exploring whether lim f(x)=L as x approaches c)