This is a Canonical Question about network mapping software.
Are there any good network mapping tools for creating a visual representation of the network?
I'm talking about an automatic mapping tool that will do a scan of the network and make a map for me.
The Dude does a good job.
To create a textual representation of the network, use nmap, if you want a GUI use Zenmap GUI
As far as creating a visual representation of the output, you're going to need to do that manually (e.g. dia, Visio, OmniGraffle).
You might consider the Weathermap project. This generally works best in conjunction with some of the larger RRD based monitoring tools which provide auto-discovery and performance monitoring.
There is an app I use called "kiss the net" that scans and generates a pdf network map for you.
It's actually based on a server monitoring/management product called "The Dude" made by mikrotik.
http://www.mikrotik.com/download/KTN_1.0beta1.zip Requirements: Windows XP/2000/2003/Vista with Internet Explorer 6 or better.
Note: the dude is the full version of the software that someone else has listed here. This one simply generates a network map along with a table of machines it discovers.
I'm not sure if you're looking for an automated solution, but OmniGraffle is a great Mac application for making flowcharts and the like. It comes with a "stencil" for electronics that you might find on a network.
EDIT: Well, you now changed your post to say that you'd like an automated solution, but I'll leave my post here. If you find a program that can scan a network and export GraphViz files, then you can use OmniGraffle to make pretty graphs.
HP's Network Node Manager performs automatic discovery of your network, and can provide a graphical output of what it has discovered, along with links between devices. I don't know if it supports exporting that data to an editable image format; in any case, most people I've talked to have always preferred manually created diagrams as they tend to be clearer and better laid out.
I've never played with it much but I've heard good things about netdisco:
Tideway Foundation is a tool for automated network discovery and application modeling that can draw visual representations of your network. You can download a free virtual appliance from their website.
Note: I used to work for Tideway and still have an interest in the company.