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.
GitSpo - Measure & engage your open-source community
moreofit.com - Moreofit is the first (and best) website similarity search engine.
wwwhere - Curated list of tools and resources for people who make websites
Libre Projects - A directory of free & open source hosted web applications and alternatives to many proprietary...