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.
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.
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
GitSpo - Measure & engage your open-source community
SimilarSites - Easily find more sites like those you like.
JSter - JSter is a community-driven catalog of the most useful javascript libraries.