The Cardboard Computer is a free (Public Domain) do-it-yourself circular slide rule that you can print out and put together at home or in a classroom. It's available in two versions: a basic version that can do basic ratios, multiplication, and division, and an advanced version that adds more features, including square and cube roots.
There is a full user guide available, and you can use the virtual Cardboard Computer below to experiment (drag the wheels and cursor around with your mouse or finger). There are also random exercises with animated solutions for both the basic and advanced versions.
Enjoy!