CommentMailer is a WordPress plugin that generates and sends e-mail to your blog’s commenters to tell them that you
have replied his/her comment on your blog’s comment system. It adds extra options on your blog’s comment form to select
which commenters you are replying to. Submitting the reply comment will trigger the e-mail generation function thus
send them automatically. You can customize your own notification e-mail or use the default one.

Free Download: Download this wp-plugin