An Excellent Geogebra animation
(This one is beautiful!!)
Another great Geogebra animation
Very cool Geogebra animation
(Very cool, but not all that useful)
Video 1
Video 2
Good Video using Protractor
(Very detailed and easy to see)
Video 3 using Protractor
Video 4 using Protractor
Video
(Very clear instructions)
Video 5 using Protractor
(Really clear directions)
Video 6
(Great notes on the history of this concept - a little deep, but good to know)
Video 7 using Protractor
(Clear instructions)