Works in 1.8**

define-method.rb

module Yay
  def self.included klass
    klass.instance_eval do
      singleton = class << self
        self
      end
      
      singleton.send :define_method, :add_a_method do |awesome_name|
        singleton.send :define_method, awesome_name do
          puts 123
        end
      end
    end
  end
end

class Foo
  include Yay
  add_a_method :stuff
end
 
Foo.stuff
#=> 123