GeoGebra is software for interactive geometry. Astonishingly is it free, astonishing as it is such of such high quality and is so well supported. There are many resources available on the Internet. I have created a Diigo list of resources which includes many examples and tutorials.
(Click on the green links for each item, then back on your browser if you wish to return to the list.)
Alternatively use the LiveBinders version.
Note that if you have not used GeoGebra before the Introductory book is very helpful. Also, see in particular Guillermo Bautista’s Mathematics and Multimedia Blog for numerous tutorials from beginner to advanced and the GeoGebra YouTube Channel.
I have included GeoGebra in my Top >10 Mathematics websites.