GeoGebra "is a free and multi-platform dynamic mathematics software for schools that joins geometry, algebra and calculus."
The simplest way to get started is just to press one of these:
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Optimized for geometry:
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Optimized for algebra:
If you use it a lot you may want to
download and install it.
If GeoGebra doesn't work, make sure you have Java 1.4.2 (or later) installed and active in your browser.
Check out some of my GeoGebra toys.