In the world of RSS feeds, one thing missing from the beginning has been support for real-time notifications. Until recently, any feed reader (Feed My Inbox in our case) has to manually go check a feed for updates on a regular basis. That is, unless it is being notified through a pinging service.
We've been testing "real-time" emails on Feed My Inbox for a while. Anyone that sends us an email (help(at)feedmyinbox.com) and requests to receive their updates in real-time rather than once per day can try it. The problem is that our system only gets around to checking a feed roughly every 3-4 hours for updates, so it is not truly real-time.
Over the last several months, two extensions to the RSS protocol have been made available, which allow real-time "pushing" of new feed content to third party services (like ours). They are RSS Cloud and PubSubHubbub. We plan on supporting both, but decided to start with RSS Cloud.
We deployed support for RSS Cloud a couple of weeks ago. Already over 2,000 feeds in our system are cloud-enabled, with that number growing every day. Cloud-enabled feeds will "push" new feed content to our service immediately, allowing the updates sent to subscribers to be truly real-time.
Dave Winer is the guy behind RSS Cloud and literally wrote the book on the RSS specification almost nine years ago. Wordpress is the first major supporter, and all 7.5 million blogs hosted on Wordpress.com are already cloud-enabled. If you self-host a version of Wordpress, there's a plugin to help.
How do I know if my feed is supported?
Look at your feed's source code. Cloud-enabled feeds have a tag like this at the top: