There are many Whois sites. Each registrar maintains a Whois database with registration and contact information for all of its own customers. In addition, the TLD registry maintains a database that allows one to find out which registrar has a particular domain registration.

To find the registrar for a particular domain name, use the NSI Registry Whois at http://www.nsiregistry.com/. This is the authoritative registry for .com, .org and .net gTLDs. If you are looking for a ccTLD domain registration, you will have to go to the appropriate Whois registry for that TLD. These are listed at http://www.iana.org/cctld/cctld-whois.htm.

There are several Whois sites that will pull up the domain owner's data directly. This information should be double-checked through the appropriate registrar database since each registrar is the authoritative search site for the registration information about its own customers. Try https://www.betterwhois.com or http://www.uwhois.com.

To search on any string within a complete URL, use Netcraft at http://www.netcraft.com/. For example if you search on ".com.py" you get EVERY domain name registered under that TLD. Should you know, however, that Netcraft only returns active domains. It is meant to tell you what type of server the domain is using - so if a site is not connected to a server the domain does not show. Nevertheless it is a very useful search tool.