nwdiag generates network-diagram image file from .diag file. .diag file is similar to DOT file (graphviz’s).
You can get diagrams like this.
Very useful, like Graphviz but specifically for network diagrams. You can probably obtain the same result with Graphviz with the appropriate amount of dot magic, but nwdiag makes this very easy. Other useful graph types in sister projects: http://blockdiag.com/en/index.html