Whoisology - Connected database of whois records and whois history
ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...
Bulk Whois API - Bulk Whois API is a provider of parsed WHOIS data in JSON with simple API, competitive pricing, top notch support, and more.
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.
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
NsLookup.io - A simple and powerful tool for querying DNS records