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.
Product Hunt - A website that lets users share and discover new products
BuiltWith - Find out the technology behind websites
Alternative.me - Welcome to alternative.me, the source of better software alternatives. Finding suitable software was never easier.
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.