DomainTools - Domain name research, including comprehensive Whois Lookup, Reverse Whois Lookup and Whois History.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
BuiltWith - Find out the technology behind websites
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
PublicWWW - source code search engine
JsonWhois.io - JsonWhois is the leading cloud hosted Whois API.