PubSubHubbub is an open protocol for distributed publish/subscribe communication on the Internet. It generalizes the concept of webhooks and allows data producers and data consumers to work in a decoupled way.
PubSubHubbub provides a way to subscribe, unsubscribe and receive updates from a resource, whether it's an RSS or Atom feed or any web accessible document (JSON...).
The current version of the spec is 0.4. Please, read it here.
Open hubs are provided by:
Several other publishing platforms, like Wordpress, include their own hubs.
If you're looking for tutorials on how to get started with PubSubHubbub, check the links below: