gistfile1.txt

module UsefulStuff

  module ClassMethods
    def thing_count
      @thing_count ||= 0
    end
    def set_count(val)
      @thing_count = val
    end
  end

  def self.included(base)
    base.extend ClassMethods
  end
end

class Foo
  include UsefulStuff
end

class Bar
  include UsefulStuff
end

Foo.set_count(4)
Bar.set_count(5)

p Foo.thing_count
p Bar.thing_count