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.
Whoisology - Connected database of whois records and whois history
NsLookup.io - A simple and powerful tool for querying DNS records
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.