http://www.lysator.liu.se/~alla/dia/
Dia is designed to be much like the commercial Windows program 'Visio'. It can be used to draw many different kinds of diagrams. It currently has special objects to help draw entity relationship diagrams, UML diagrams, flowcharts, network diagrams, and simple circuits. It is also possible to add support for new shapes by writing simple XML files, using a subset of SVG to draw the shape.
It can load and save diagrams to a custom XML format (gzipped by default, to save space), can export diagrams to EPS or SVG formats and can print diagrams (including ones that span multiple pages).
I'd be very happy if someone could port this to OSX or give me comprehensive instructions on how to install/compile the requred components