Scott Reilly

This plugin adds a checkbox to the profile page for users which allows them to opt into receiving a notification email whenever a comment is made to the site.
If a comment goes into moderation, only users who have the ability to manage comments on the site will receive the moderation notification email.
By default, all users of the site have the ability to subscribe to notifications about comments. A filter is provided to facilitate use of code to customize the feature’s availability to users.
Note: a “user” is a person with an actual login account for the site. The plugin does not facilitate permitting visitors who do not have an account on the site to be able to subscribe to all comments.
Links: <a href="http://coffee2code.com/wp-plugins/optin-comment-notifications/" rel="nofollow ugc">Plugin Homepage</a> | <a href="https://wordpress.org/plugins/optin-comment-notifications/" rel="ugc">Plugin Directory Page</a> | <a href="https://github.com/coffee2code/optin-comment-notifications/" rel="nofollow ugc">GitHub</a> | <a href="http://coffee2code.com" rel="nofollow ugc">Author Homepage</a>

Optin Comment Notifications

Checkboxes on user profiles to receive emails about new comments. Like it says.

Exactly what you want

Allows users to opt into receiving a notification email whenever a comment is made to the site.

Alex Moss

Google+ comments was introduced in April 2013 to blogger.com users. This plugin lets you add Google+ comments to your WordPress site!
Simply install the plugin and follow the instructions on the Settings page.
<ul>
<li><a href="http://3doordigital.com/wordpress/plugins/google-plus-comments/" rel="nofollow ugc">Google+ Comments</a> Plugin homepage.</li>
<li>More <a href="http://3doordigital.com/wordpress/plugins/" rel="nofollow ugc">WordPress Plugins</a>.</li>
</ul>
You can also insert the comment box as a shortcode into any post, page or template and use your own settings for each time you do it! Simply use the shortcode [gp-comments]

Google+ Comments

The Google+ Comments WordPress plugin makes it easier for you to setup, administer and customise Google+ comments from your WordPress site.

Optin Comment Notifications vs Google+ Comments