When troubleshooting networks, when there is an IP-address conflict for example, it is sometimes useful to lookup the vendor of the mac-addresses. This way you can identify the brand of the conflicting host.
A tool for doing this is found here. It contains information about both ethernet and bluetooth manufacturers.
When you need to know what your current IP-adress on the Internet is, there is a tool:
This page displays your current IP-adress, your reverese DNS name, how your browser identifies itself and if you are accessing the Internet through a proxy server. This is the information servers that you visit on the internet will see about you.
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.