First, Web browsers brought us one screenful of the Internet at a time. Then search engines made it possible to find whatever we wanted. Now subscription-based "feeds" allow us to pick and choose the content we want -- news summaries, podcasts, event calendar updates, photos, video -- and sit back while it's automatically delivered to our homes and offices.

This latest innovation in Web-based technology has burgeoned over the past year, especially in the form of a syndication standard called RSS (Really Simple Syndication). Now, marketers and advertisers are waking up to its possibilities.

Yahoo is experimenting with ways of inserting ads into feeds, which should help businesses see how effective a marketing tool it can be. In November, Yahoo upgraded its Publisher Network so that bloggers and other online publishers could include ads in their feeds. And companies such as FeedBurner and Feedster are tracking the number of people subscribing, reading, and re-syndicating feeds, to give publishers, and ultimately advertisers, information on where their ads are going and how to better target their audiences.

One attraction of RSS ads may be that feed syndication is still in the "early adopter" phase -- meaning that feeds' audience members are not typical Web surfers. For the most part, they have actively set up feed readers and subscriptions -- they've "raised their hand and said ‘I'm interested'," as Ben Fox, senior product manager in Yahoo’s search marketing division, puts it. "You know from a marketing standpoint that they've invested in their content."

Yahoo's RSS ad scheme exploits this fact. For instance, if a blogger writes about digital camera technology and products, and publishes an RSS feed through Yahoo, he or she can send subscribers an ad from Canon with the blog entry's headline. The blogger earns a bit of cash and Canon gets to target a self-selected niche audience -- that's likely to include people shopping for a camera.

Feeds are chosen by publisher, such as Reuters, or by subject matter, such as Washington politics, so subscribers' interests are clear to advertisers, explains Feedster president Chris Redlitz. Another advantage to advertising through feeds, both for the subscriber and advertiser, is that feeds can't be spammed by outsiders, unlike e-mail ads that overwhelm an Inbox, Redlitz says. And since a subscriber doesn't have to use e-mail to get a feed, there's less risk to a user's privacy, he adds.

At this early stage, companies haven't yet settled on the best way to advertise through feeds. But detailed data about their subscribers could help. FeedBurner has introduced a new technology for publishers to track their feeds, using software that sorts the way people view feeds and follows the content flow when it is redistributed.

Of course this benefits advertisers, too, who want to know whether their ad spending is effective. "Over the past 10 years, people have been able to track site traffic and the paths people take to the site; but being able to track feeds is a whole different model," says Eric Lunt, chief technology officer for FeedBurner. It's not just how many people subscribe to feeds, he says, but how many people are repeatedly exposed to the feeds and how many actually read them.