OpenGrok - OpenGrok is a fast and usable source code search and cross reference engine.
BuiltWith - Find out the technology behind websites
GrepCode - GrepCode.com is a code search engine built by developers for developers to search and browse open...
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Krugle - Krugle is the complete enterprise solution for search targeted to the development organization.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.