Subscribe to Comments Reloaded 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 or suspend all notifications. It solves most of the issues that affect Mark Jaquith's version, using the latest WordPress features and functionality. Plus, allows administrators to enable a double opt-in mechanism, requiring users to confirm their subscription clicking on a link they will receive via email.