Riccardo Conte

This plugin implements a “like/dislike” rating system for comments. In the setting page you can choose a “compact / splitted” mode that show ratings like sum or separately.

Zaki Like Dislike Comments

This plugin implements a "like/dislike" rating system for comments

Scott Taylor

Most ratings plugins contain too much code: inline JavaScript, messy markup, weird CSS. Rate is simple, hardly intrusive, and completely overridable.
A Post/Page/Custom Post Type’s rating is the average of all comment ratings. A user can leave a rating when commenting, and change that rating inline after leaving a comment (if logged-in or Cookie’d).
Don’t be afraid to play around and extend the code: drop a <code>rate.css</code> file in your theme directory and mine won’t even load (by default, Rate stars are transparent with a white border, so you can use <code>background-color</code> to set your stars’ colors)!
<pre><code><?php 
// in this version, you need to insert these functions into your theme for ratings to appear
// you don't have to use the_rating(), the comment_rating() will work by itself, but the_rating() will not

// for a Post, Page, or Custom Post Type (average of all comment ratings)
the_rating();

// for a comment
the_comment_rating();
?>
</code></pre>
Read More: http://scottctaylor.wordpress.com/2010/11/30/new-plugin-rate/

Rate

Most ratings plugins contain too much code: inline JavaScript, messy markup, weird CSS. Rate is simple, hardly intrusive, and completely overridable.

Zaki Like Dislike Comments vs Rate