SourceForge - The Complete Open-Source and Business Software Platform.
BuiltWith - Find out the technology behind websites
openDesktop.org - The website openDesktop.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
OSOR - OSOR is the Open Source Observatory, a project to provide a framework for developing and executing autonomous observations.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.