Dia

Graph Drawing/Structured Diagram tool

http://www.gnome.org/projects/dia/

specifically positioned as competition for MsVisio

http://en.wikipedia.org/wiki/Dia

http://www.advogato.org/proj/Dia/

Dia is written in C and has support for plugins written in Python.

file format is XML, can export to SVG

for MacOs X

• doesn't sound like fun to compile it

• but Darwin Ports sounds less painful, so may try that