BuiltWith - Find out the technology behind websites

Stackshare - StackShare is a comprehensive website that gives its users the chance to organize and share their technology stack with the rest of the community.

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.

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