glg_subscribe_comments allows your visitors to subscribe to the discussion of an article.
It add a checkbox below the comment form. If the visitor check it when he comments, he will receive a notification every time a new comment is posted.
The plugin works well if you have the moderate_comments preference activated.
You can customize the content of the notification mail and the format of the subject.
Feel free to send feedback, bug, and spelling or grammar mistakes at firstname.namewithoutblank (at gmail).
Language packs are available in the forum thread and on the information page on my website.