Send a email to author automatically while someone reply his comment.
So this plugin apparently hasn't been updated in a while, but I have it on my blog and it works well. It's easy to install and use. When someone leaves a comment on your blog, if that comment is replied to the original commenter will get a short email telling them there's been a reply to their comment. You can customize the email (subject and body) as well, but I think the default settings are fine. Not everyone who comments on your website will check back to see if someone's replied to their comment, so this plugin comes in handy.
Very happy with the plugin. Easy to set up and use and does exactly what it should. Now I know that readers will see the reply I leave to their comment.