Git.io OS X Service
Download the Service and open it. OS X will ask whether you want to install it to your
~/Library/Services folder or open it in Automator.
When installed correctly you'll see a "git shortener" item in the Services System Preferences pane.
Go to Safari and surf to github, select any URL, right click to bring up the Services menu and select git shortener. You'll see Growl notification displaying the selected URL and shortly after that, the shortened Git.io URL. The shortened URL is added to the clipboard, so you can easily paste it wherever needed.