Gymno online journal
http://www.schuetziana.org/
Cacti pdf library
http://www.cactuspro.com/biblio/
A couple of links
In a similar vein, a new online-only journal began with issue 1 being published just yesterday:
http://www.cactusexplorers.org.uk/journal1.htm
http://www.cactusexplorers.org.uk/journal1.htm