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I'm thinking something on the site, similar to the way you can star questions. You should be able to star tags or groups of tags and when something new happens in that set (an edit to a question or answer, a new answer to an existing question, or a new question), you get notified on the site.
Yes, you can use an RSS feed, but if I were to do that, my RSS feeds would probably triple. I would like to be notified on-site of events such as these.