Donncha O Caoimh (a11n)

Adds the referrering link from which a commenting user came from to comment notifications.
Note: if you’re using Jetpack Comments the referrer information is lost and unfortunately this plugin won’t work correctly.

Comment Referrers

Adds the referrering link from which a commenting user came from to comment notifications.

Mark (podz)

With this plugin you can see how people were referred to your site, ideal if you want to see how people are finding your blog.
All you need to do is select how many referrers you wish to show and the title of the widget.

SeeReferrers Plugin

With this plugin you can see how people were referred to your site, ideal if you want to see how people are finding your blog.

Comment Referrers vs SeeReferrers Plugin