Often, diagramming is necessary to document a project, or for a business proposal etc. Diagrams in the documentation plays a vital role in communicating the details to the engineering team. E.g. Flowcharts always help to understand the problem better. There are popular commercial diagramming tools such as Microsoft Visio, which is one of the industry leader. Visio has lots of features but expensive. If you are looking for a free Flowchart, Network or a generic diagramming tool, the Dia which is Free and Open Source software is worth taking a look. It is cross-platform, it can run on Windows, Linux etc. It supports various kinds of diagrams;
Civil and much more.
A sample flowchart created using Dia
Simple and Easy User Interface
Dia has a decent GUI (user interface) and pretty intuitive. It has lots of in-built graphic images for creating various diagrams. It is very easy to design flowcharts and other diagrams. Arrows and lines can automatically attach to other diagrams and managing a large diagram is quite easy. Diagrams can be exported to various formats such as PNG, JPG, WMF etc.
Object Sheet Editor
An UML Diagram
How to add a custom shape to Dia?
If you need to have more graphic images, Refer to this tutorial.
Quality of Diagrams
Well, quality of the graphic images/icons are good but compared to other tools, they are a little archaic. Icons are very basic and richness of the graphic quality could be improved. But, overall, Dia is a great tool and it is pretty reliable too. It can work as an alternative to Microsoft Visio.