I could see this working in a way similar to the following:

For the Site:

• A site registers with the service.
• The site sets up events on the service which will occur on their site and for which they would like to send out notifications. For example:
  • When a new news article is added under any topic.
  • Set variables in the event, such as which topic (as opposed to any topic).

For the User:

• A user registers with the service as a user.
• They identify sites and events on the service they would like notifications for, and subscribe to those notifications. e.g. Any news items about Hard Knitting from ripmyknitting.com.
• Select notification methods. These could in various forms, such as messaging or e-mail.

In Action:

• When a configured event occurs on the site:
  • The site sends notification to the service only.
  • The service receives notification from the site.
  • The service identifies all users subscribing to that event, and sends out the configured notifications via the available methods (e.g. e-mail) to all subscribing users.

I would see this service being useful for the sites and the users. I am not sure what the business model for the service provider may be. Perhaps advertisements and revenue from those in the messages or e-mails sent.