foobar.rb

module Widgets
  def self.extended(base)
    class << base
      attr_accessor :widgets
    end

    base.widgets = {}
  end

  def add_widget(name, widget)
    widgets[name] = widget
  end
end

class Foo
  extend Widgets
end

class Bar
  extend Widgets
end 

Foo.add_widget('foo', :bar)
Bar.widgets['foo']