Move the red dot on the parabola and the gradient function (a linear function) is graphed below it. Very neat. From Manipula Maths.
http://www.ies.co.jp/math/java/calc/x_diff/x_diff.html

This applet also draws the graph of the 2nd derivative.
http://www.ies.co.jp/math/java/calc/x_2nd/x_2nd.html