GitHub Visualizer - Enter user/repo and see the project visually
Refined GitHub - Browser extension that makes GitHub cleaner & more powerful
Gource - Gource is a software version control visualization tool.
GitZip - Download or create a download link for a GitHub project folder/sub-folder or file.
SyntaxDB - Easily look up programming syntax for multiple languages
GitHub Hovercard - GitHub Hovercard provides neat hovercards for GitHub.