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.
If you subscribe to a lot of RSS feeds, you may find yourself wanting to view only a subset of the content. You can organize by folder, but sometimes searching the feeds is the quickest way to get at what you seek. A new service from Q-Sensei is now bringing that feed search power to your applications via its Q-Sensei FeedBooster API.