Commit Together by Github - Now add co-authors to your commits
GitSpo - Measure & engage your open-source community
GitHub Marketplace - Tools to build on and improve your workflow
Devo - Devo delivers real-time operational & business value from analytics on streaming and historical data to operations.
Refined GitHub - Browser extension that makes GitHub cleaner & more powerful
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.