Check out the <a href=”http://www.netscantools.com/spmapmain.html”>Switch Port Mapping Tool</a> from NetScanTools. This question has also been asked before on this site so you should be able to search for <a href=”http://itknowledgeexchange.techtarget.com/itanswers?s=automated+network+diagram&searchtype=IT+Answers”>previous answers here</a>.
Another good tool if the switches are Cisco is the Cisco Network Assistant tool. You can get that on the Cisco website.
You can also use Cisco Packet tracer , a software with which you can try creating simulation of real world scenario.(Download it from Cisco site)
There is also a free add on to Visio available here : http://www.bestsecuritytips.com/xfsection+article.articleid+184.htm