Artzone
http://www.artzone.gr/index.htm
Asiaphoto
http://www.asiaphoto.com/zone
Basical information
http://www.elsorta.com.ar/sistemazonal.htm
Zone system course and virtual simulator
http://www.cicada.com/pub/photo/zs/toc.html
To put this links here don't mean that I agree all the things that this courses explain.