Whois Lookup
Find the owner of a domain name, and furthermore find other domains hosted by the same person.
Official links for Whois Lookup
Whois Lookup Review
This tool is useful if someone violates your copyright or you want to contact the owner of a domain for any other reason.
See an example for the query "google.de":
The tool also knows which other domains are registered on the same owner. Unfortunately they do not give away this information for free. E.g. for Google the report would cost about $49,000. Incredibly priceworth, huh :)?