Do you use domaintools.com? If yes then http://whoisrequest.org will be probably useful for you. Unlike DomainTools, WhoisRequest
don't require subscription or any payments at all and offers almost the same features as DomainTools.
So what does WhoisRequest.org offer?
One of the best tool for me is the Domain NS History checker. This tool allows you to see historical DNS data for any com, net, org, info, biz and us domain.
Of course you can do whois queries the returned information is structured nicely. WhoisRequest also offers Reverse IP checker tool, you can
browse domains using certain IP address.
I highly recommend http://WhoisRequest.org and I'm pretty sure that you'll like it to.