Shraga - Cross platform, cross browser, multi purpose tool for web developers, designers & webmasters
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
PublicWWW - source code search engine
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
BuiltWith - Find out the technology behind websites