Dia is a gtk+ based diagram creation program released under the GPL license. It is free and open source general-purpose diagramming software.
Dia is 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).
I feel Dia is in a state where it can be actively used. Many features are implemented and the code is quite solid and mature.
Development
Dia was originally created by Alexander Larsson but he moved on to work on GNOME and other projects. After him James Henstridge took over as lead developer but also moved on to other projects.
Dia is maintained by a group of developers Hans Breuer, Lars Clausen, Steffen Macke, Sameer Sahasrabuddhe.
Dia is written in C, and has a system of extensions that can be written in Python.
