Screaming Penguin - Source Does Matter

nmap is a GREAT tool. nmap can basically tell you anything about anything when it comes to TCP/IP. Since TCP/IP is the main protocol suite in use all over the web and in most LANs that makes nmap a very valuable tool indeed.

nmap is available for just about any *nix platform.

CrossNodes has a very high level article about "auditing your LAN with nmap." While the article gives no real detail, about anything, its on the right track in that admins should do this and should be aware of every service and port that is accessible on their networks.

Check the linked article for elementary nmap usage and of course checkout what you can tell about a machine with netstat (man nestat, I prefer netstat -nap to see all listening processes, ports and connections and the daemon that is making the port available).
  Audit Your LAN Before the Bad Guys Do with nmap