This geometry applet is being used to illustrate Euclid's Elements. Above you see an icosahedron, that is, a regular 20-sided solid, constructed according to Euclid's construction in proposition XIII.16. Another example using this Geometry Applet illustrates the Euler line of a triangle. Here's how you can manipulate the figure that appears above. If you click on a point in the figure, you can usually move it in some way.
Source files are available on the page.
It intersects the opposite side at its foot. The following illustration shows the Nine-Point circle. Source code is available on the page. It stores exactly 3 edges per point and 2 pointers per edge, yet supports all the standard operations. This applet illustrates it.