http://www.teacherlink.org/content/math/interactive/flash/derivativetool...

The derivative machine can be viewed on the web or it can be downloaded and run as a standalone program (recommended for the classroom as it can viewed full screen). Students can see graphically how the secant approaches the tangent as delta-x approaches 0. Very nice!

http://clem.mscd.edu/~talmanl/MTH1410F07/Pictures_091007/
From the website created by Lou Talman: In each of the following pictures, which curve is the original function, and which curve is the derivative?