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.
Ruby Toolbox - Explore and compare open source Ruby libraries
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.
GitNews Web - Trending repositories from GitHub, HackerNews & Reddit
Hackster - Hackster is a community dedicated to learning hardware.
JSter - JSter is a community-driven catalog of the most useful javascript libraries.