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.
Refined GitHub - Browser extension that makes GitHub cleaner & more powerful
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.
GitZip - Download or create a download link for a GitHub project folder/sub-folder or file.
GitSpo - Measure & engage your open-source community
GitHub Hovercard - GitHub Hovercard provides neat hovercards for GitHub.