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.
UserBit - A full-stack qualitative research platform for UX & product teams.
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
Hackster - Hackster is a community dedicated to learning hardware.