WhoAPI - WhoAPI is an API service for mass requests of domain information like domain registration availability and structured whois data in JSON.

DNSChecker.org - Check DNS Propagation worldwide. DNS Checker provides name server propagation check instantly. Changed nameservers so do a DNS lookup and check if DNS and nameservers have propagated.

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