BuiltWith - Find out the technology behind websites
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
TechsFound - Find Running Technologies on Website
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
Similar Tech - SimilarTech offers a Sales Insights platform which helps companies uncover their ideal market by crawling the sourcecode of over 300M sites.
Webspotter - Webspotter is a lead-generation tool and a website database. With its help, you can learn about all the technologies installed on a particular site