module Combo
  def self.included(item)
    (@items ||= []) << item
  end
  def self.hit
    (@items || []).each { |i| i.hit }
  end
end

# pouet.rb
class Pouet
  include Combo
  def self.hit
    puts "Pouet"
  end
end

# prout.rb
class Prout
  include Combo
  def self.hit
    puts "Prout"
  end
end

Combo.hit