SimilarWeb - SimilarWeb.com is a website analysis tool that gives you analytics information for any website.
Whoisology - Connected database of whois records and whois history
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WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Quantcast - Quantcast Corporation is a digital marketing company that provides free audience demographics measurement and delivers real-time advertising.
Bulk Whois API - Bulk Whois API is a provider of parsed WHOIS data in JSON with simple API, competitive pricing, top notch support, and more.