Traceroute, looking glass and whois servers
In order to analyze and troubleshoot networking issues over the Internet, there are a number of traceroute and looking glass servers availble. By using these, you can analyse the routing paths to your network. You are able to see how your network is beeing reached from different parts of the Internet.
A collection of traceroute and looking glass servers is found on http://www.traceroute.org/.
On Geektools you will find a collection of useful tools like traceroute and a whois lookup (who owns a certain IP-address).
SamSpade.org has a whois gateway for looking up who owns a certain IP-address.