LibHunt - LibHunt tracks mentions of software libraries on relevant social networks. Based on that data, you can find the most popular projects and their alternatives.
BuiltWith - Find out the technology behind websites
AlternativeTo - AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
JSter - JSter is a community-driven catalog of the most useful javascript libraries.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.