Allows you to register handlers for exceptions in your controllers like:

class ApplicationController < ActionController::Base
  class NotAuthorized < RuntimeError;end

  register_exception NotAuthorized, :not_authorized

  private

  def not_authorized
    flash[ :error ] = 'You are not authorized to perform the requested ' +
    'action.'
    render :text => '', :layout => true, :status => 401
  end
end

In this example, you could raise an ApplicationController::NotAuthorized exception anywhere in the course of processing an action, and instead of just getting an error report or a 500 error in the browser, the user would see the page with your error message and the browser gets a 401 status code.