gistfile1.rb

module MyModule
  def self.included(base)
    base.extend(ClassMethods)
  end
  
  module ClassMethods
    def my_method
      @myvar ||= 0
    end
    
    def my_method=(value)
      @myvar = value
    end
  end
end

class MyClass
  include MyModule
end

puts MyClass.my_method
MyClass.my_method = 5
puts MyClass.my_method

class AnotherClass
  include MyModule
end

puts MyClass.my_method
puts AnotherClass.my_method