Justin Shattuck has created a new plugin that was made to help create communication between a commenter and the post author.
Here are the details from his site:
Visitors love interaction, especially a personable approach at gaining their attention and spiking their interests. Increasing the overall readership of your blog revolves primarily around content, accessibility and presentation. In an attempt to gain return readers and possibly subscribers, I present Comment Relish.
What is it
Comment relish is a WordPress plugin developed to send an e-mail message to users who comment on your website who have never commented before. The message dispatched to the user is defined within the plugin’s preferences. Numerous tags have been integrated to allow for information to be included in the message easily (I.E.: timestamp, author name, comment, ETC.).
So if you are looking to thank first time commenters on your site, check out Comment Relish.