Common action for all the controllers.

maybe i'm completely confused about what you're trying to do, but
application-controller says the following:
# Filters added to this controller apply to all controllers in the
application.
# Likewise, all the methods added will be available for all
controllers.

that's right, but you didn't mention you were trying to write a
plugin.
in that case try to write it like this:

# my_plugin.rb:
module MyPlugin
  def my_action
  end
end

# init.rb:
ActionController::Base.send :include, MyPlugin

hope i didn't forget anything.

sorry, just checked it. i gotta be like this:

  # my_plugin.rb:
  module MyPlugin
    def self.included(base)
      base.extend(ClassMethods)
    end

    module ClassMethods
      def my_action
        # ...
      end
    end
  end

irb(MyOtherController):001:0> MyOtherController.methods.include?
('my_action')
=> true