appable_plugins - Adds the ability for plugins to share models, controllers, and helpers with your Rails application. Classes are automatically mixed in with your own or other plugins' models/controllers/helpers.
Wiki http://wiki.pluginaweek.org/Appable_plugins
Announcement
Source http://svn.pluginaweek.org/trunk/plugins/rails/appable_plugins
Development http://dev.pluginaweek.org/browser/trunk/plugins/rails/appable_plugins
Enjoy, from the PluginAWeek team 
Alex,
Thanks for the comment
I would have to disagree with you, though, on it being included in core. I think it's a little too "magical" and invasive to warrant core inclusion. That's part of the reason engines were never considered for core. Plugins, as provided in the framework, were meant to be very simple and clean-cut. I actually wrote a little piece on when plugins should be included in the core (アコムの審査は甘いのか？他社のカードローンと審査難易度を比較検証).
As far as releasing plugins that use this type of functionality, you'll have to take the same approach you would with anything else. Generally, what I do is require the plugin from within my lib/plugin_xyz.rb file. If the plugin isn't there, then the application will fail to initialize, warning the user that a required file is missing. You would then have to point them to a place to get your plugin's dependencies (in this case, appable_plugins).
There're currently ideas floating around on building a framework on top of existing components (generators/gems) for installing plugins along with their dependencies. Hopefully something will come out of this in order to help ease the process of distributing plugins that depend on other plugins.
P.S. Most of the plugins at PluginAWeek will eventually be released as
gems as well. We're working on it
Alex Wayne wrote: