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.
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.
UserBit - A full-stack qualitative research platform for UX & product teams.
GitSpo - Measure & engage your open-source community
Hackster - Hackster is a community dedicated to learning hardware.
JSter - JSter is a community-driven catalog of the most useful javascript libraries.