I've thought for quite a while that RSS is one of the most important, and often overlooked technologies around for building interesting services. Overlooked because it has potential for much more than just the currently-widespread blog-type syndication of (new) content.

I think there's an amazing potential in delivering all kinds of information to users via RSS feeds personalized to their individual profile - whether based on recommendations, locale, activities, or like this new service FeedCycle, by serializing access to previously  published stuff in a sequence that makes most sense to the recipient. Think "my pals say Galactica is really cool, but I missed its first season - so I'd like to watch it two episodes a week to catch up". Or, one of those continuing novella-based stories many magazines publish.