I have been meaning to post this for awhile. Among other things, I recently wrote a Processing sketch that uses Carnivore to sniff the packets on a local network, then does a whois search on the IP address to look up the physical address attached to them. The sketch then displays the address, and plots it on a map. The more packets that come from a location, the brighter the dot.
Since this is a Processing sketch, I could have done an embedded applet for this entry, but Carnivore requires some changing of permissions on the local machine to do the packet sniffing. And well, you can't do that from a web page.
The code for this is here, but I will warn you now that it is undocumented. And it comes with no warranty, use at your own risk, etc. Enjoy!