Verisign has been granted a patent protecting the lookup of domain names. The patent protects the act of performing several look-ups at once, or "performing a multitude of searches simultaneously, transparent to the user. "Specifically, the improved query server searches for an existing domain name records in various domains and then displays the results in a formatted manner, thus eliminating the need for a user to perform individual searches."
DNS lookup patent | The Register
Any sites allowing you to search for a domain name without an extension, then listing all the possible open domain names are in violation of this patent.
Time to start the search for prior art!