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.

ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...

Whoisology - Connected database of whois records and whois history

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