Dougal Campbell

This plugin allows you to automatically add Gravatars for commenters to your 
theme, if your theme does not already support them.
According to the Gravatar.com website, Gravatars are Globally Recognized 
Avatars, or an “avatar image that follows you from weblog to weblog 
appearing beside your name when you comment on gravatar enabled sites.” 
You register with the Gravatar server, and upload an image which you will 
use as your avatar. The gravatar image is keyed to your email address, so 
that it is unique to you.
This plugin will display gravatars for the people who comment on your posts. 
You do not need to modify any of your template files — just activate the 
plugin, and it will add gravatars to your comments template automatically.
<h3>Credits</h3>
Based on a code snippet from Matt Mullenweg: 
 http://photomatt.net/2007/10/20/gravatar-enabled/ 
 http://pastebin.ca/743979
Props to David Potter for pointing out that Gravatar normalizes email 
addresses to lowercase before hashing with MD5: 
 http://dpotter.net/Technical/index.php/2007/10/22/integrating-gravatar-support/

Easy Gravatars

nice

Add Gravatars to your comments without modifying any template files. Just activate, and you're done!

blueinstyle

Display your top commenters or authors in a widget, or you can display anywhere on your blog by pasting this code into your theme: <code><?php if(function_exists('jme_top_contributors')) { jme_top_contributors(); } ?></code>
Version 1.4 includes many user requested features. Check <a href="http://justmyecho.com/2010/07/top-contributors-plugin-wordpress/" rel="nofollow ugc">plugin webpage</a> for details on the update.
<ul>
<li>List your top commenters or authors with the option to display their Gravatar, and several other options.</li>
<li>Choose from 2 formats of the widget, with complete control of styles via css.</li>
<li>Exclude users from the list by email address.</li>
<li>The list uses a cache system for improved performance. List updates only when a post or comment is added, or options updated.</li>
</ul>
<h4>Extra Feature</h4>
<ul>
<li>Add a special Icon next to each of your Top Commenter’s name in their comments to give them a little special recognition for being a regular contributor.</li>
</ul>
Support and Feature request forum at http://justmyecho.com/forums/

Top Contributors

Display your top commenters or authors in a widget.

Easy Gravatars vs Top Contributors