Embedded Sage Cells
One of the neat things about Sage, the open-source computer algebra system is that you can easily embed it into any web page. Here is an example of some code that can be evaluated to plot a function and it's derivative. For more information about Sage: http://sagemath.org. To learn how to embed Sage into your web page: http://aleph.sagemath.org/static/about.html.