GitZip - Download or create a download link for a GitHub project folder/sub-folder or file.
Commit Together by Github - Now add co-authors to your commits
Refined GitHub - Browser extension that makes GitHub cleaner & more powerful
Commit Print - Posters of your git history
GitHub Hovercard - GitHub Hovercard provides neat hovercards for GitHub.
gitbird - So, I don't always remember to tweet what I do, but commit my code often, and what do users love more than your product?