If you want to share complete modules between your applications, you can look into Rails Engines (http://rails-engines.org/). I believe this will change (or maybe already has) in the next version of Rails, as it is evolving to a core framework where you add plugins for specific functionality (e.g. the in_place_editor will become a plugin in Rails 2.0). You might look into the mephisto blog code, I believe it implements extensibility by adding extra modules.
Peter De Berdt