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.
openDesktop.org - The website openDesktop.
SimilarSites - Easily find more sites like those you like.
SourceForge - The Complete Open-Source and Business Software Platform.
Product Hunt - A website that lets users share and discover new products
OSOR - OSOR is the Open Source Observatory, a project to provide a framework for developing and executing autonomous observations.