Polygonal Portal
Thinking about ways to make a nice polygon spiral. This one is created by rotating and dilating the point that determines the first rotated polygon. (Using matrix exponentiation to rotate around the origin.) I love the spiral and 3-D effect of the transformation.
There is a later sketch, http://www.geogebratube.org/material/show/id/155782, that generalizes this one in a few ways. Including tools to make compound portals!