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.
Siftery - Share products your company uses and get personalized recommendations
BuiltWith - Find out the technology behind websites
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!
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.