I was just impressed with the Cinderella software:
http://www.cinderella.de/tiki-index.php
The examples in the gallery are very nice (interactive, work in
browser). However, what really impressed me was the article on the
theoretical background:
http://doc.cinderella.de/tiki-index.php?page=Theoretical+Background
Some very nice thoughts there, in particular the use of complex numbers
in geometry to get rid of special cases. Go read it already!:)