GitHub Repository Size - Browser extension to show file and repo size in GitHub
Refined GitHub - Browser extension that makes GitHub cleaner & more powerful
GitZip - Download or create a download link for a GitHub project folder/sub-folder or file.
Sawia - Find trending open source projects to contribute to
GitHub Hovercard - GitHub Hovercard provides neat hovercards for GitHub.
Trello Bookmark - Store your bookmarks as Trello Cards