PubSubHubbub. Say that three times fast. If you're not familiar with PubSubHubbub, it's an extension of the RSS and Atom protocol that allows the feed to push content out when it's updated. That's just the sort of technology we should see from check-in APIs like Foursquare and Gowalla.
Our API directory now includes 53 feeds APIs. The newest is the App.net API. The most popular, in terms of mashups, is the Google Ajax Feeds API. We list 45 Google Ajax Feeds mashups. Below you'll find some more stats from the directory, including the entire list of feeds APIs.
There are a number of ways of delivering data in real-time but until recently it has looked like PubSubHubbub, with the backing of Google, was going to be the preferred method. However, the past couple of weeks have seen a couple of interesting developments which could indicate that the developer community may actually prefer HTTP Streaming.