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
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.
Devo - Devo delivers real-time operational & business value from analytics on streaming and historical data to operations.
JSter - JSter is a community-driven catalog of the most useful javascript libraries.
wwwhere - Curated list of tools and resources for people who make websites