CabriJava

Cabri have released a beta version of CabriJava, allowing interaction
on the Web. A free copy may be obtained via the CabriJava page.

At the moment, each beta expires at the end of the month of release.
Details of using CabriJava may be found on the official how to page.

The figure on the right illustrates Pascal's Theorem.
You can vary the figure by dragging the point A.

As further examples, I've implemented figures illustrating

• Pappus's Theorem
• Brianchon's Theorem
• Desargues Theorem
• The Three Tangents Theorem
  
• A proof of the Theorem of Pythagoras.
  

Full code for the first three figures, and more about Pappus, Brianchon
and Desargues can be found in the Classical Theorems pages.

See also the pages on inversive geometry (using CabriJava).