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Check out jsxgraph: http://jsxgraph.uni-bayreuth.de/wp/

I wrote up a Newton's Method example a few years ago and it still holds up well: http://www.programmingmath.com/

If you want to create class material, have a look at http://challengeu.com - they've built an entire service around providing a platform for teachers to share content. It's pretty cool! (Disclaimer: I used to work with them)

I don't have a lot of experience with it, but Bret Victor's http://worrydream.com/Tangle/ come to mind. Hope it helps!

Thank you everyone. I'll check the suggested options.