Viscosity Explorer is a simple site that allows you to explore the concept of viscosity. It consists of two containers and two steel balls. You select the liquid you want in the container, then drop the ball and observe what happens. You can also change the temperature of the liquid. Students can easily compare the viscosity of two different substances side by side.
It's a great site for science classes, and much faster, cheaper, and less messy than doing this for real. I would, however, want to at least demo the real liquids and pour them from container to container to show how the flow of the fluids is different based on the viscosity.
PhET - excellent, free, virtual labs and simulations for science