Rhys Wynne

This plugin quite simply lets you know where the users entered your site when they leave a comment. A lot of people who joined the do-follow movement are listed on sites which are a target for comment spammers. To those who suffer comment spam can now find out the page they come from, and block accordingly.
This plugin uses a cookie to track the commenter, so if the user hits the first page first, they will still be tracked.

InComment

Adds an extra "referral" note to the bottom of comment forms, so you could see where people come from comment. Useful for finding resources  …

Nikolay N.

Simple plugin that will improve your onsite SEO a bit and make all links from the comment section dofollow, if they point to your own domain. Other links that point to external websites will remain nofollow. WP DoFollow Comment Links don’t have any configorations – simply activate it and that’s all.
It also works with WordPress installations that reside in a subfolder. For example, WordPress installed in example.com/blog, separate from the main website example.com.
If you have any questions or suggestion, hit me up… nnikolaev [at] sematigo.co.uk

WP DoFollow Comment Links

The plugin automatically makes all the comment links to your own domain dofollow, while all external links remain nofollow.

InComment vs WP DoFollow Comment Links