I think you may have been correctly thinking of domaintools. When they were just whois.sc, I beleive you could do the reverse whois searches for free as long as you signed up for a free account. Sadly, that is no longer available.

I did find http://www.whoisxmlapi.com, which seems to have a much lower price point thann domaintools, but I've not tried it, so can't speak to how effective or accurate it is.

Ken