Superfeedr

 Superfeedr  provides a variety of ways to access real-time feed updates, including a popular PubSubHubbub-0.4 compliant hub. Publishing and subscribing to their PuSH hub is a free service, and they also offer a paid service (with a free tier for low volume users) that handles all the polling, bookkeeping, and logistics of monitoring feeds of all kinds (XML, HTML, JSON) for updates. This includes feeds that do not ping a Superfeedr hub and feeds that do not support PuSH at all.

In June 2016, Medium acquired Superfeedr.

IndieWeb Examples

 * Many of the personal sites listed on PubSubHubbub delegate to a Superfeedr hub.
 * Bridgy subscribes to the Superfeedr API to get realtime updates for hosted blogs on Wordpress.com, Tumblr, and Blogspot, that it sends webmentions on behalf of.