Whois search tools

Who owns a domain name?

Domaintools Who.is    
What's on the server?
See also webhosting.info and spyonweb and yougetsignal
Website Informer
allows various fields and keywords to be searched. See also Weboar
Domaintools (subscribers) Netcraft report
Click site report for WHOIS, then "netblock owner".
IP Explorer
Domaintool's tool for checking similar or adjacent IP addresses.
Reverse Internet
Also allows you to check neighbouring domains.
Hurricane Electric
Investigate a domain name and its IP address block.
Really useful tool for investigating a domain. Finds its subdomains etc.
Specialist IP address tools
Visual Traceroute 
Geographically trace the actual server containing a Web page or allocated an IP address. See also YouGetSignal
Locate an IP address down to the users home town! See also Max Mind
Various IP location & whois tools from an Austrian site. Sometimes finds details that others miss.
International IPs
Find which regional authority handles IP address allocation for a country. 
Email tracer
(use with caution!) See also IP-Address.org's email tracer
DNS Stuff
Various tools to probe a server.
IP Explorer
Domaintool's tool for checking similar or adjacent IP addresses.
Investigate an email server
See also reversemx
Ahref - Investigate by links and keywords    
Historical WHOIS How popular is a domain?
Domaintools (subscribers) Who.is archive (free) Webrank stats  
International WHOIS/IP research
National Authorities
International bodies that issue country code domains. If WHOIS fails, go to the source!
See also IANA
Generic Domains
Authorities for domains that don't have a country code or
standard top level domain.
British domain authority who
oversee the .uk domain names
The TV Corp.
WHOIS for the .tv domain. See also DotFM WHOIS for the .fm domain
IP authority for the Asia Pacific region.
The IP issuing authority for Europe, have the best WHOIS lookup for European IP addresses.

IP authority for for North America 
for Latin America & the Caribbean,
and AFRINIC for Africa