The English version of the Altermundus site showcases the beautiful work of the author, Alain Matthes, using packages that he created: . They're built on PGF and Tikz and, like the sagetex package, are some of the most important packages for anyone teaching math---especially the tkz-euclide package for geometrical drawings. Since the packages are built on Tikz it's possible to use Tikz code, too. The problem (at least for me) is that the documentation is in French which makes going through it a bit slower than I'd like. I've also organized the material differently to help me as I learn more about this package.
tkz-euclide package (geometrical diagrams)