WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
DomainTools - Domain name research, including comprehensive Whois Lookup, Reverse Whois Lookup and Whois History.
Whoisology - Connected database of whois records and whois history
APISpot.io - Bulk WHOIS and Domain Names API on steroids, Parsed and in JSON, Domain Availability Check, Domain and WHOIS Lookup
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.
Domain Details - Displays Server Type, Headers, IP Address, Location Flag, and links to Whois Reports.