APNIC - Asia Pacific Network Centre
ARIN - American Registry for Internet Numbers
ASCIO - Ascio Technologies, Inc
ATNIC - Austrian (.at) Domain Registry
AUNIC - Australian (.au) Information Centre
BENIC - Belgium (.be) Domain Registry
BRNIC - Brazilian (.br) Domain Registry
BULKREGISTER - Bulkregister.com
BUYDOMAINS - DomainDiscover
CHNIC - Swiss (.ch) Domain Registry
CIRA - Canadian (.ca) Internet Registration Authority
CRNIC - Costa Rican (.cr) Domain Registry
CORE - Core Internet Council of Registrars
DENIC - German (.de) Domain Registry
DIRECTNIC - DirectNic.com (Intercosmos Media Group, Inc)
DOMAINBANK - Domain Bank, Inc
DOTFM - Dot FM (.fm)
DOTREGISTRAR - DotRegistrar.com (IHoldings.com, Inc)
DOTSTER - Dotster, Inc
ENOM - Enom, Inc
ESNIC - Spanish (.es) Domain Registry
GODADDY - Go Daddy Software, Inc
INFO - .info Domain Registry
INTERDOMAIN - InterDomain, S.A.
INWWCOM - Internet Names Worldwide (Melbourne IT, LTD)
KRNIC - Korea (.kr) Network Information Center
LACNIC - Latin America and Caribbean Domain Registry
LUNIC - Luxembourg (.lu) Domain Registry
MXNIC - Mexican (.mx) Domain Registry
NETSOL - Network Solutions
NEULEVEL - NeuLevel (.biz) Domain Registry
NICLINE - NicLine.com (Arsys Internet, S.L.)
NICSE - Swedish (.se) Domain Registry
NLNIC - Netherlands (.nl) Domain Registry
NuNames - NuNames, LTD (.nu)
OPENSRSNET - OpenSRS.net (Tucows, Inc)
ORG - .org Domain Registry
REGISTERCOM - Register.com, Inc
RIPE - RIPE (European) Registry
SRSPLUS - SRSPlus (TLDS, Inc)
UKNIC - United Kingdom (.uk) Domain Registry
WSNIC - Western Somoa (.ws) Domain Registry
For a more complete list of Country Specific whois servers, please visit IANA's ccTLD reference page.
Some other GTLD (Generic Top Level Domains) whois servers, for extensions such as .aero, .edu, .int, .mil, .museum and .pro can be found at IANA's GLTD page.