PubSubHubbub is a simple way to let people know in real-time when your blog is updated. PubSubHubbub is quickly gaining adoption and is already being used by Google Reader, Google Alerts, FriendFeed and more.
WordPress Plug in: http://wordpress.org/extend/plugins/pubsubhubbub/
Another Plug in is PuSHPress: http://wordpress.org/extend/plugins/pushpress/
