Notify.io integrates with Growl and other local notifiers (as well as email, Jabber, Twitter, and webhooks) and provides a dead-simple API for any web developer to send real-time notifications to their users.
You can think of Notify.io as a web-level Growl system. It empowers users with a consistent, controllable way to get notifications, and it provides developers with a simple, consistent way for sending those notifications.
Notify.io is an open platform for notifications. It’s still in a pre-alpha state, but it already has several useful notification sources. Last Thursday I built Feed Notifier, which uses PubSubHubbub to give you real-time desktop notifications of Atom and RSS feed updates.