Logarithmic Spirals
A sketch made as a GeoGebra companion to Vi Hart's video on spirals, to let anybody try to experiment with spirals by pitch and by how many fit around a circle.
Her excellent video: http://www.youtube.com/watch?v=ahXIMUkSXX0
The wikipedia primer on logarithmic spirals: http://en.wikipedia.org/wiki/Logarithmic_spiral