Whoisology - Connected database of whois records and whois history
IsHostedBy.com - IsHostedBy is a website where you can lookup where certain websites are hosted.
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Website Informer - Website.informer: complete data lookup & free aggregated report on any domain including whois, visitors, IP & DNS details, competitors, owners, etc
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.
Whats My DNS - Instantly check your domain names current IP address and DNS record information against multiple name servers located in different parts of the world.