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.
TechsFound - Find Running Technologies on Website
BuiltWith - Find out the technology behind websites
PublicWWW - source code search engine
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.