Great Software for Plane Geometry
Interactive plane geometry software
Interactive plane geometry software were made popular first by Geometer's Sketchpad at least back to 1994. It is a wonderful tool made possible by technology. Such program allows one to construct plane geometry drawings dynamically, much in the way of Greek's “Ruler and Compass”. For example, draw a triangle with lines bisecting the 3 angles. They intersect in a point called the incenter of the triangle. (which is also the center of the largest possible circle inside.) Now, you can drag the corners of your triangle to change its shape, and all your drawings change accordingly. Because of such dynamic power, a few theories on plane geometry have been found because of such software.
In the 1990s, there's only Geometer's Sketchpad and Cabri Geometry. In late 1990 came Cinderella, written in Java. But since about post 2000, there's a explosion of interactive geometry software. As of 2007, there are more than ten of them (counting only quality ones), many of them are free. They are all similar. They are all of high quality. Almost all of them can be saved as Java appletes so that it can be placed on the web for students to use without needing extra software. Their features differ slightly. Several of them are FreeSoftware. The following is a short commentary of the ones i've actually used (except otherwise stated).
GeoGebra
GeoGebra is a interactive geometry software. Written by Markus Hohenwarter, started in 2001.
Geometry Expressions
Cabri II Plus
Cabri II Plus, developed in France. Cabri is one of the early interactive geometry software in the mid 1990s. Cabri has a hardware TI calculator version. A particular feature that is first done by Cabri is that it has conics sections as a primitive. That is, you can have 5 points that defines a conic section, that is any of parabola, ellipse, hyperbola. Dragging any of the points will change the conics section. Any other figures build on the conic section will of course change accordingly. This makes Cabri a excellent choice for doing projective geometry.
Geometer's Sketchpad
The Geometer's Sketchpad, created by Nicholas Jackiw. This is the one that started dynamic geometry software in the early 1990s, and is still excellent. macOS Mac OS 9 Windows
CaRMetal
CaR
Spirograph
[Spirograph By Nathan Friend. At http://nathanfriend.io/inspirograph/ [2016-12-29 best spirograph emulation in JavaScript. source code at [2016-12-29 https://github.com/nfriend/inspirograph ] ]]