Alternato - NOTE: Not available in English.
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.
AlternativePedia - Discover free, open source or simply better alternatives for your software based on our research...
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.
Aventrix - Aventrix is a website that helps you discover new and fun activities.
GitSpo - Measure & engage your open-source community