Update: There has recently been an interesting conversation about the term, "headless" Drupal. The suggestion has been made that "decoupled" Drupal makes more sense and I tend to agree. Regardless of whether you prefer "headless" or "decoupled", it's still an exciting approach to building websites.
If you're a frontend developer or designer that has grumbled about the challenges of Drupal theming, you no doubt applauded the announcement that the Twig template framework was being added to Drupal 8.
Fortunately, there are some folks that are already doing exactly that and sharing the results of their work. It's something called "Headless Drupal" and it's an approach that uses Drupal as a backend content repository and REST server.
A REST server makes it possible for other applications to read and update data. The typical case is that Drupal is used to store and manage content and it then provides that data to your app built with Angular, Backbone, Ember, or whatever. If that's not entirely clear, don't worry. The links below will help sort it out.
Headless Drupal Resources
Headless Drupal Manifesto - This is great place to start. It succinctly answers the question of why anyone would want to do this sort of thing.
Headless Drupal Group - A group on Drupal.org devoted to sharing ideas, discussion and experiments around the topic of headless Drupal.
What Is Headless Drupal? - A great introduction to the topic from OSTraining.
Headless Drupal, One form at a time - This is a great post from Amitai Burstein that demonstrates some of what this approach has to offer for the creation of frontend user interfaces.
Headless Drupal Demo - Working Code and Call to Action - Great resource from Josh Koenig of Pantheon.
Headless Drupal - Inline edit - Another good one from Amitai.
Here’s Drupal - Tonight on the Tonight Show with Jimmy Fallon - A case study of headless Drupal in action on a very high profile site.
Not "Headless", But Related and Useful
Move logic to the front end with AngularJS - Juan Pablo Novillo Requena of Lullabot talks about using AngularJS together with Drupal. It's not a decoupled Drupal approach, but is a useful and related look at using a frontend framework.
If you know of some other resources, please share them in the comments below. I'd love to check them out.