Why not just buy a TI-89, 92, or a Voyage 200 calculator?. Because they've been discontinued, you can get 92's very reasonable off of eBay. I don't think there is a freeware differentiator available to my knowledge. Could be, though.
Thanks for this new category... it will be useful.
Now I'm looking for a program that can find derivatives and antiderivatives, similar to wolfram's integrator but with a "differentiator" as well. I have the virtual TI emulator with a ti92 skin but it's too cumbersome and ugly. Thanks in advance.