Drupal-simple-subs is a project mainly written in ..., it's free.
Simple subscriptions to node comments attached to nodes
This module is an extremely simple subscriptions module which allows users to subscribe to a node's comments (specifically, NodeComment comments).
Users receive an email upon subscription and then a single email for each new comment published to the discussion. A user can subscribe either by clicking a link that can be placed on the node's display, or via a checkbox in the nodecomment form.
Users can unsubscribe by clicking either a link displayed in the node or the link present at the bottom of the default email included with the module.
The main topic this code covers is the use of the drupal_mail() function, which due to its flexibility is somewhat cumbersome to use for simple purposes.
As with most modules, this code also shows examples of using custom permissions, different kinds of hook_menu() callbacks, and using the Form API to change how form submissions are handled.