I've always wanted a tool that would be able to dig into the publicly available whois databases and find information... There's a website (http://www.domaintools.com) that has a pay-for feature much like this, but obviously, for those of us who need to do this on a regular basis, paying $100 per search isn't quite cost-effective.
Perhaps an extension onto your already popular and successful DNS Analyzer, you could allow for lookups into the whois database, as well as the DNS database.
Just an idea - the diagramming piece of DNS Analyzer would be AWESOME if it could work with whois the same way - essentially, I put in someone's email address, and I get a visual representation of the domains they're associated (as a contact) with and the registrar of the domain, and perhaps even the NS records for that domain as well.
I find that so many companies end up having multiple domain names out there, and there's just no easy way to get a listing of all of them. This tool could help eliminate that issue...