http://mathplotter.lawrenceville.org/mathplotter/mathPage/riemann.htm

A lovely Geogebra java applet from Miguel Bayona, The Lawrenceville School, Lawrenceville, NJ. It demonstrates the lower sum, upper sum, left sum, right sum, midpoint sum and trapezoidal sum for a function of your choosing. Loading the applet can take a while, so be patient.