ViewWhois is an online tool to search whois of a domain or an IP. It also works as a domain availability checker. It currently has 400+ TLDs supported for domain whois lookup, while ip whois lookup can be used for any IPv4 IP Addresses.
With viewwhois you also get the IP of the domain, and Google PageRank of the website. While an IP lookup also returns the hostname of that IP.
It would not be wrong if this tool is called ip to domain and domain to ip converter, a whois lookup tool, or a domain search tool, with an easy to use interface.