I'm a big fan of Tapbot's Twitter client Tweetbot because it gives me a consistent Twitter experience across my electronic devices. As I was investigating the various settings in the application, I noticed that the URL Shortening service had a selection for a custom shortening endpoint that meets these specifications.
So, I decided to whip up a little Rails application so that I can run my own URL shortening service, and thus crisco was born. It runs easily on for free on Heroku.
You will need to set the CRISCO_SECRET environment variable.
Crisco uses the indispensable devise authentication library, and it should be multi-user ready, though I have only used it in a single user enviroment. Feel free to expand on that if you have a need for it, I would be happy to get pull requests to make crisco a better piece of software.
I have released crisco under the zlib/libpng license, so go nuts.