RIPE
http://www.ripe.net/cgi-bin/whois

InterNIC (.COM .NET .ORG .EDU)
http://www.internic.net/whois.html

ARIN
http://www.arin.net/whois/

APNIC
http://www.apnic.net/apnic-bin/whois.pl

US Department of Defense Network Information Center (.MIL)
http://www.nic.mil/cgi-bin/whois

US General Services Administration (.GOV)
http://www.nic.gov/cgi-bin/whois

CERT's database of TLDs and WHOIS servers
http://www.cert.org/tech_tips/whois_by_domain.html

NORID's list of TLD registries
http://www.uninett.no/navn/domreg.html