Subscribe to Comments is a plugin that allows commenters on your blog to check a box before commenting and get e-mail notification of further comments. It is one of the most popular WordPress plugins out there for the simple reason that it helps foster a community around your blog by encouraging commenters to come back and stay engaged in the dialog.

Features

• Rudimentary security
• Recognition of subscription status
• Themeable subscription manager
• Ability to let users change notification e-mail address
• Easy integration into comments form
• Subscription without leaving a comment
• Customization
• Internationalization

Pricing: Free
Requirements: WordPress 2.2+
Source: http://txfx.net/code/wordpress/subscribe-to-comments/