This is a new plugin, EMailSubscription and it comes as a better built follow-up to my EMailDiscussion addon.
It allows users to subscribe to certain categories of the forum (receive e-mail notification when a new discussion is posted) and it auto-bookmarks newly created discussions so that users will receive an e-mail notification each time a new comment is added to a discussion. Users can change their subscription settings by visiting their profile page and then choosing EMail subscription in the side panel.
Also, there is a Dashboard->EMailSubscription button that allows admin users to subscribe all users to all content (newly created).
I consider this plugin useful for some small communities, where the people don't visit the forums that often, but read their emails regularly, thus enabling them to be more interactive with the forums.
TODO for next release: set by default new users to be subscribed to all content.