BuiltWith - Find out the technology behind websites
Microlink - Extract structured data from any website
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
CRX Extractor - Get any Chrome Extension source code. Learn and hack!
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
bloop - Code-search engine for developers