Desmos is a free interactive web-based graphing calculator.
This summer, I created two Desmos graphs to demonstrate integrals and derivatives to a class of teenagers in a visual, interactive, intuitive way. I am sharing these graphs here so that you will use them to demonstrate integrals and derivatives, but I also challenge you to create and share your own Desmos graphs to demonstrate all sorts of cool stuff. Desmos, itself, is intuitive and easy to use, but I've also commented each graph so that you can understand each step.
link to integral demo:
link to derivative demo: