[ Thanks to Lee Schlesinger for this link. ]
Nmap is a powerful utility for scanning your network
and discovering all kinds of information about who is on it and
what they’re doing. You can discover used and unused IP addresses,
hostnames, services, and operating systems, and their versions
– information that can help you monitor who is on your
network, and lead you to unsafe or unauthorized servers.
Nmap is included in all Linux distros, and the project also
offers binaries for Mac OS X and Windows.