OpenGrok - OpenGrok is a fast and usable source code search and cross reference engine.
BuiltWith - Find out the technology behind websites
Sourcegraph - Sourcegraph is a free, self-hosted code search and intelligence server that helps developers find, review, understand, and debug code. Use it with any Git code host for teams from 1 to 10,000+.
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.