Productz.com - Crowdsourced products database
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.
Versus - Find popular alternatives to anything in a jiffy
Alternative.me - Welcome to alternative.me, the source of better software alternatives. Finding suitable software was never easier.
Diffen - Compare anything to find similarities and differences. Read, improve or create comparisons.
Product Hunt - A website that lets users share and discover new products