Setalist - The easiest way to make awesome lists for the internet

BuiltWith - Find out the technology behind websites

Hunter TechLookup - Create lists of websites by technology for free

Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.

Techmap.io - Techmap is a workplace search engine to find companies using technologies (e.g., Rails, PHP, Java, etc.) that are used internally. Finally, a "Google for Workplaces" that helps people find a place to work with technologies they love!

WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.