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.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
PublicWWW - source code search engine
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
W3Techs - W3Techs provides information about the usage of various types of technologies on the web.