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
BuiltWith - Find out the technology behind websites
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
TechsFound - Find Running Technologies on Website