Product Comparisons - Compare any 2 software products
BuiltWith - Find out the technology behind websites
TestBox - Test drive your software options before you buy
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
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.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.