A long time ago I worked on building an enterprise information portal product. At the end of my stint I designed and implemented the subscription and notification system. My mantra at the time was "You don't have to find the information, the information finds you." This is now more and more possible with a wide variety of services. Google monitors the Web and has email notifcations for search results, pubsub.com monitors blogs and provides RSS feeds of the results, and I finally have my old SearchAlert.net service running again.
What SearchAlert does is monitor searches and send notifications when new results are found. One of the advanced features of SearchAlert are to let a user choose between searches against news items or the full web. A new feature added recently is to send Web notifications in addition to email notifications. The Web notfications send XML (or whatever) to whatever receiver you choose, and pre-defined templates support the Atom API, the Weblogger API and others.