Geogebra is a comprehensive FREE dynamic geometry program that can also be used to teach coordinate geometry and calculus.

http://www.geogebra.org/cms/

This Word document is a beginner's guide to Geogebra. Geogebra is a free dynamic geometry program available in both Windows and Mac format from
http://www.geogebra.org/cms/

This document gives a brief introduction to Geogebra in the context of circle geometry.
Thanks to David Crewe for sharing.