Stefan M.

This plugin will set all Trackbacks and Pingbacks automatically to DoFollow Links.
The user comments will get after <code>X</code> comments from a unique mailaddress automatically DoFollow status. All other comments have NoFollow, except the Admin enable manually the DoFollow Status. The status which is set manual (if DoFollow or NoFollow) will overrides the automatical process. So, you have the control if someone gets sooner the DoFollow status, or never maybe. Of corse, the automatism can be disabled to do the hole work manually.
You get an support automatism, that you don’t need to check daily, but have the full control power.
Additonal this plugin validated all DoFollow URLs and will notice if there are broken links. Broken Links are very bad the Rank in the Search Engines (SEO).
Home Page of the Plugin: <a href="http://blog.murawski.ch/2010/09/wordpress-manual-dofollow-plugin/" title="IT Blögg - WordPress Manual DoFollow Plugin" rel="nofollow ugc">IT Blögg – WordPress Manual DoFollow Plugin</a>
If you have a wish for new functions, please contact me.

SMu Manual DoFollow

SMu DoFollow has many DoFollow Options (Manual or Automatism) and included URL Validator (Manual, WP-Cron or Cronjob).

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  …

SMu Manual DoFollow vs InComment