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.
PubSubHubbub has become the standard protocol for real-time RSS and Atom feed subscription and delivery. But not everybody wants to host their own PubSubHubbub hub in the same way that hardly anybody hosts their own website, and why cloud services in general have become so popular. Guzzle Ayup has entered the market to offer a hosted PubSubHubbub hub service.