Reverse whois tool or service

Hi, im new here , so big Hello to everybody.
Im looking for reverse whois tool, i know there is expensive tool on domaintool... (cant put urls here ) but its way to much expensive if you wanna get new raport ofter.
Maybe you know where i can find something good for checking reverse whois , domains registered by one company name .. etc. Thanks

Is reverse whois same as rdns?

i think its not same thing. Reverse whois should show you all domains belongs to one registrant. ( one company .. )

Not reverse whois but also very helpful http://reverseinternet.com/