I've read about HMVC (Hierarchic Model View Controller) and it's flexible structure.
Have a look at this picture:
http://techportal.inviqa.com/wp-content/uploads/2010/02/MVC-HMVC.png
I wonder if the Rails 3 plugins are the answer to HMVC in Rails 3?
Based on the comments to Toby's answer it seems that you would like to be able to have MVC apps used as a component within a new app. Rails Engines (See http://rails-engines.org) provides this functionality. You simply install the engines gem and place apps in vendor/plugins and its modles/views/controller are all accessible.
This does not really conform to HMVC where the controllers in the new app delegate to other controllers. But like Toby I do not see the advantage of that.
What is nice about the Engines approach is that you can over ride any of models in the plugin by just adding a version of the model to the new apps app/model folder (same applies for views and controllers)
I have overidden app/views/layouts to give my Authentication app/plugin the same look and feel as the application it is included in.
For Rails 3 Railtie takes the place of engines and is officially supported (and actually used - Action Mailer is a Railtie plugin. I have not used it myself yet though.
Check it out at http://edgeapi.rubyonrails.org/classes/Rails/Railtie.html
A nice write up on it is also here http://www.igvita.com/2010/08/04/rails-3-internals-railtie-creating-plugins/
