Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
BuiltWith - Find out the technology behind websites
Competera - Competera is a price tracking tool for performance-oriented eCommerce retail and brands of all sizes worldwide.
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
Kompyte - Track & analyze competitors in real time.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.