Beherit

Sends an simply email notification to the comment author, when someone replies to his comment. No configuration, support WordPress translation process.

Simple Comment Notification

Sends an simply email notification to the comment author, when someone replies to his comment.

Aaron Axelsen

Overrides the <code>wp_notify_moderator</code> function located in pluggable.php. By default, wordpress does not notify a comment author of a moderation 
event. This plugin will look at all users in the blog, and notify anyone who can moderate the plugin, along with the admins and the author 
of that post. This has been tested and does work with WPMU as well.