Code Climate for Google Chrome - Code quality and test coverage, directly in GitHub
Gource - Gource is a software version control visualization tool.
Devo - Devo delivers real-time operational & business value from analytics on streaming and historical data to operations.
The GitHub Matrix Screensaver - Latest commits from GitHub visualized Matrix-style
Git Random - Chrome extension to replace new tab with random github user
Octohunt - Search to find developers in your city on Github.