plugin.rb

class Plugin
  def self.plugins
    @plugins ||= []
  end
  
  def self.inherited(klass)
    @plugins ||= []
    
    @plugins << klass
  end
  
  # Usually a good idea for debugging if you have lots of methods
  def handle_command(thing)
    raise "#{self.class.name} doesn't implement `handle_command`!"
  end
end

class Dog < Plugin
  def handle_command(thing)
    puts thing
  end
end

Plugin.plugins.each do |plugin_class|
  plugin_class.new.handle_command('test')
end