Here's a brief beginner's starter-pack for performing a
WHOIS search on a domain (only .COM, .NET, .EDU and .ORG at the moment):
Go to the internic whois page at http://www.internic.net/whois.html and type the domain you're interested in.
It will either list the full details (name and address of the registrar), or will list a WHOIS server that _does_ know. (for example, if it says 'whois server: whois.networksolutions.com', browse to 'whois.networksolutions.com', go to the 'whois lookup', and try again)
You can usually find out the name, address and phone number of who registered the domain. Handy, huh?