The Subscribe to Comments plugin by Mark Jaquith allows anyone who posts a comment to your blog to elect to receive an email when any further comments are made.
This can really help to foster an active discussion from your blog readers, as people who have posted a comment will know if someone replies to a point they made, and can return to post further comments. Let’s face it, once you’ve read a post and commented, most people don’t have time to keep returning to check for any new comments.
Do yourself a favour, download Subscribe to Comments and install it (it’s trivially easy to install).
The default setup will add the “Notify me of followup comments via e-mail” tickbox to the comment form. If you want to offer the ability to subscribe to comments without having to post a comment first, just add the following just after the closing </form> tag of the comment form:
<?php show_manual_subscription_form(); ?>