I know that most major social services automatically shorten URLs for you now, but there are still times I personally need to create a short url, often for the purpose of preserving query strings when submitting to various services. This is the OS X System Service I use to quickly create short urls.
It will scan selected text and shorten any url it finds. Additionally, if you configure an iTunes affiliate string, it will intelligently append it to any itunes.apple.com links prior to shortening.
The only configuration required is a Bitly API key and username1. If you have a bit.ly account set up, you can find this on your advanced settings page under “Legacy API Key.” Open the Service in Automator (you can just double click it to install and/or open it) and you’ll see the configuration section at the top of the script action. You can also add a Bit.ly custom domain if you have one set up, and clear out or customize the iTunes affiliate link info (mine is in there by default as an example).
Once configured, just select a url or any text containing one or more long urls, right click and choose “Bitlyize” from the contextual menu.
Fix: The JSON blob that was showing up in output has been corrected.
Update: The download now also includes a Bitlyize to Clipboard command that will output the results to the clipboard and work on uneditable text fields.