WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Whoisology - Connected database of whois records and whois history
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.
WebCargo - Optimize your workflow by offloading it to us. Useful API's with seamless integration that scale to your needs.
Active Whois Browser - Browse all information for an IP address or domain name with a single click!
ICANN WHOIS - The official ICANN WHOIS search for domain lookups.