WhoAPI - WhoAPI is an API service for mass requests of domain information like domain registration availability and structured whois data in JSON.
ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
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.
Complete API - All the APIs you need under one roof: SMS, Geolocation, Whois and Currency APIs now available!
NsLookup.io - A simple and powerful tool for querying DNS records