Dia – Opensource Diagram Creation Program for Linux, Unix and Windows
http://live.gnome.org/Dia

Dia is a GTK+ based diagram creation program for Linux, Unix and Windows released under the GPL license. Dia is roughly inspired by the commercial Windows program ‘Visio’, though more geared towards informal diagrams for casual use. 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 many other diagrams. 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 a number of formats, including EPS, SVG, XFIG, WMF and PNG, and can print diagrams (including ones that span multiple pages). They feel Dia is in a state where it can be actively used. Many features are implemented and the code is quite solid and mature. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog.