DomainTools - Domain name research, including comprehensive Whois Lookup, Reverse Whois Lookup and Whois History.
Whoisology - Connected database of whois records and whois history
CubDomain - CubDomain.com is the site to check website's statistics, Information, Whois Record, Hosting...
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Online Domain Tools - Online-Domain-Tools.com makes your online life easier and happier.
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.