rssCloud

 rssCloud  is part of the RSS specification, it allows publishers to notify subscribers when there are updates to a feed.

IndieWeb Examples
Malcolm's site https://unicyclic.com provides updates to subscribers, and will also subscribe to notifications from other sites that support rssCloud.

Wordpress
Wordpress.com supports rssCloud. See: https://josephscott.org/archives/2009/09/rsscloud-for-wordpress/ and https://josephscott.org/archives/2009/10/rsscloud-wordpress-plugin-update-0-4-1/

Scripting.com
Dave Winer supports rssCloud and has provided some documentation at: http://walkthrough.rsscloud.co/