JsonWhois.io - JsonWhois is the leading cloud hosted Whois API.
DomainTools - Domain name research, including comprehensive Whois Lookup, Reverse Whois Lookup and Whois History.
EZwhois - Whois domain information such as registrar, domain status, expiration date, and name servers.
ICANN WHOIS - The official ICANN WHOIS search for domain lookups.
DNSdumpster.com - Open source domain intelligence lookups for security recon
ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...