I've setup a new plugin on some of the Wordpress blogs we publish which sends an email to those who have commented on a post when a new comment is added. Its opt-in of course, and very low volume. We have our blog comment spam under control, so that's not a problem.
Turns out I really like this feature, as well as a similar feature I've been working on in PhunkyBB - the ability to subscribe to topics and forums.
I usually can't stand impersonal e-mail like this (especially when the email doesn't contain the message, only a link back to the site), but I have to admin that its growing on me.
On the other hand, I've always liked impersonal e-mail that provides status updates for online purchases and travel arrangements.