(Please bear with me as I edit this and add hardlinks.) This is an excellent program, especially for high school students who are starting to learn complicated geometric properties. You can construct a plethora1 of different geometric figures:
And then, you can construct other figures onto one you've already put down. Ie, you create a circle, place a point on top of it, then create a line through this point and the center of a circle.
Using this program, I've successfully constructed conic sections (parabolas, ellipses, hyperbolas) and also illustrated problems for my classes. (For those of us who have visual minds, it is very helpful to do this.)
Other things you can do: You can also do transformations on a figure: You can do all sorts of measurements. You can measure: On top of all that you can enter an equation where the measurements are the variables, and then use that to define some part of your figure.
A locus is really neat. You start with a point that is defined to be on a path (ie. on a circle or a line or somesuch) and another point that is somehow affected by the position of the first point. You can then construct a locus, which is the set of all the positions that point 2 encounters as point 1 moves along its path. It then stays there while you manipulate the figure. You can use this to construct say, a parabola, and then move the directrix and the focus around to see how the parabola changes.
You can also animate things, by telling the program to animate one point along its path and you can then see the rest of the figure bend and twist or whatever as this happens.
You can also add in labels, colors (pretty colors!) and text.
The Geometer's sketchpad also comes with a very nice and easy-to-use macro utility, it allows you to recreate a construction you made multiple times and in multiple projects. Nice!

The Geometer's sketchpad was created by Nickolas Jakiw and is © Key Curriculum Press. It works on Win32, but older versions work on Windows 3.1 as well.

1 I'm always looking for a place to use that word.

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