There's something super elegant about Writebook's use of concerns. I especially like Book:Sluggable:

module Book::Sluggable
  extend ActiveSupport::Concern

  included do
    before_save :generate_slug, if: -> { slug.blank? }
  end

  def generate_slug
    self.slug = title.parameterize
  end
end

Here's a few reasons:

• Nesting concerns in a model folder is neat when that concern is an encapsulation of model-specific functionality: app/models/book/sluggable.rb.
• Concerns don't have to be big. They do have to be single-purpose.
• Reminds me of a great article by Jorge Manrubla: Vanilla Rails is plenty. Down with service objects!