JsonWhois.io - JsonWhois is the leading cloud hosted Whois API.
DomainTools - Domain name research, including comprehensive Whois Lookup, Reverse Whois Lookup and Whois History.
WebCargo - Optimize your workflow by offloading it to us. Useful API's with seamless integration that scale to your needs.
ICANN WHOIS - The official ICANN WHOIS search for domain lookups.
ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.