PubSubHubbub is a simple, open, web-hook-based pubsub (publish/subscribe) protocol.
Decentralized and free. Anybody can run a PubSubHubbub server or anybody can use an open server.
PubSubHubbub is just a protocol, not a service, but we're running this as an open test server for anybody to use to help bootstrap the protocol. Feel free to publish to or subscribe from this hub. You can migrate in the future when you want to run your own hub server, or you can just keep using this one.
See the project for details. This presentation has an overview:
From here you can,
These legal disclaimers are here because this Hub is run by Google as a service. If you don't want to agree to these terms you should just run your own hub. The PubSubHubbub protocol is decentralized and free.