What is Dia?
Dia is a GTK+ based diagram creation program for Linux, Unix, Mac OS X and Windows released under the GPL license.
Dia is roughly inspired by the commercial Windows program Microsoft Office 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.