Note: This review addresses v0.96.1 of the software.
For those who depend on organization charts to make sense of life, Dia is a savior. A product of the Gnome Project, Dias' nearest relative in the commercial software world is Visio. Dia doesn't try to take on Visio diagram to diagram, polyline to polyline. But it provides more than enough of the usual suspects--boxes, ellipses, polygons, and sticky connecting lines and arrows--to create office diagrams, chains of authority, and electrical circuits. If you need shapes not in Dia, there are instructions on how to add custom objects.
If those tools aren't enough to for you to organize anything from the office hierarchy to your kids' Little League season, Dia has a few other tricks to help you. A good one is layers. You can create your diagram as a stack of subdiagrams, a virtual representation of drawing the diagrams on pages of clear acetate. The layers let you extend your diagram into the third dimension. (Think of the layers as separate blueprints for each story of a office building. You get to see not only how offices are laid out on each floor, but how the floors are connected by wiring, pipes and elevators.)
The best trick, though, is Best Fit. Any diagram I have ever drawn, on paper or on the screen, wound up running off the edges because it was simply too big. Best Fit takes care of that in an instant, reducing my rambling diagrams to whatever size they need to be to fit where they're supposed to be. Makes me look as it my own thinking were organized.
Note: This download takes you to the developer's site, where you can download the software. The Windows download is at the top of the first page; links to the Mac and Linux downloads are further down the page.