GitMail is a small Ruby project that gets you a list of contributors' emails from a Git repository. I made it a couple years ago and never released it, and when I did, it recoeved some discussion on HackerNews, where it reached the front page. It also has a little over 100 stars on GitHub (where it is currently my most starred project) and is the first project I ever worked on where I recieved a Pull Request from a stranger, which was a delightful feeling.
You can check out the GitHub repository for it here.