PubSubHubbub ("PuSH") is one of the key technologies for an open, real-time Web.
It uses the Webhooks pattern to let subscribers register interest in an Atom feed, and to let publishers distribute updates at time of publication, rather than at a later poll time.
(Evan "Rabble" Henshaw-Plath and Kellan Elliott-McCrea 's great talk at OSCON 2008 discusses the problems with polling many feeds at scale, and propose a pub-sub solution, albeit XMPP-based.)
The specification for PubSubHubbub was originally developed by Google engineers Brad Fitzpatrick and Brett Slatkin. They have a good video that goes over what the protocol does and why they developed it. There's also a demo from the 2009 Real-time Crunchup with similar info.
In the years since its first release, the PubSubHubbub has been remarkably widely-implemented -- on blogs and in feed-reader applications. New usage has turned up some new requirements. Among other things, defining how the protocol works for non-Atom data types (like Activity Streams JSON or even binary types), and defining how to limit distribution to particular individuals.
The original IP is licensed under the OWFa agreement.
PubSubHubbub is the key component to #OStatus.