This is cool – FriendFeed created the SUP protocol (format) for web services that produce multiple feeds. In order to reduce tons of frequent feed polling, services can offer one “SUP feed” that indicates what feeds on the service have been updated.
It works very simply. Each RSS/Atom feed includes a link tag with that feeds SUP-ID (so each feed has its own SUP-ID) and the URL of the SUP feed. The SUP feed contains a list of all the SUP-ID’s of feeds that have been updated.