Open Source / Non-Proprietary
a GTK+/GNOME diagramming tool that also supports UML (licensed under the GNU GPL)
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).
The authors feel Dia is in a state where it can be actively used. "Many features are implemented and the code is quite solid and mature. Try downloading Dia and tell us what you think of it. If you find any bugs, please report them with Gnome Bugzilla. Check out the code too, you might even want to contribute..."