You can query most WHOIS databases from domainmonster.com, but for reference the official online WHOIS for some TLDs are:

.UK - www.nic.uk
.COM, .NET, .BIZ, .INFO, .MOBI - http://www.internic.net/whois.html
.ORG - www.pir.org
.EU - whois.eu

You can find information on each TLDs WHOIS from the www.iana.org website:

Generic http://www.iana.org/gtld/gtld.htm
Country Specific http://www.iana.org/cctld/cctld-whois.htm

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Web Address Information & Resources.