Make it easy for your readers to keep up with new comments on a post.

Website: http://www.satollo.net/plugins/comment-notifier

Once you have the plugin installed and activated, the option can appear as a checkbox at the bottom of your posts like this:

When a new comment is posted, the user gets an email that looks something like this. You can customize the wording as well:

There is also a one-click unsubscribe link, in case they no longer want to receive updates.