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.
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.
Product Hunt - A website that lets users share and discover new products
Alternative.me - Welcome to alternative.me, the source of better software alternatives. Finding suitable software was never easier.
GitSpo - Measure & engage your open-source community
SaaSHub - Find and promote software that will help you grow your business or to be more productive.