This is one of those plugins that I install by default on every single WordPress site I set up.
From the author:
Subscribe to Comments is a robust plugin that enables commenters to sign up for e-mail notification of subsequent entries. The plugin includes a full-featured subscription manager that your commenters can use to unsubscribe to certain posts, block all notifications, or even change their notification e-mail address!
Unlike many plugins, Subscribe to comments does everything that its author claims. This is the plugin that I use for this site, as well as Pippin’s Pages.com, and it is also used on some other well known names, such as Pro Blog Design and WP Mods.
The only aspect of the plugin I am not found of is its subscription manager. It works very, very well, but I do wish that it was built such that it could integrate directly into a WordPress page, rather than having its own separate interface that does not match the site it is running on.
Overall, a tremendous plugin.