TwitterAuth is an awesome gem written by mbleigh, which you can check out [here](http://github.com/mbleigh/twitter-auth/).

MongoMapper is an awesome gem written by jnunemaker, which you can check out [here](http://github.com/jnunemaker).

This began as a proof of concept of putting the two together, but it is slowly becoming a full-fledged auth strategy. It's rough and undocumented now, so use at your own risk, but more features and clear documentation are coming.