HackADay - Hackaday.io is a platform for people who like to build things.
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.
Wikifactory - Engineer the future with Wikifactory. Wikifactory unifies teams in real-time, enabling efficient communication, streamlined workflows, and accelerated time-to-market.
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.
Hackster - Hackster is a community dedicated to learning hardware.
GitSpo - Measure & engage your open-source community