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!

mindutopia

Nate Reist

This plugin allows you to:
<ul>
<li>Add a photo with each user, much like featured images</li>
<li>Replaces the user gravatar with their featured photo</li>
<li>Only users levels that can upload_files in their capabilities can manage their user photo</li>
</ul>

Mindutopia User Thumbnails

This plugin gives you the ability to add user thumbnails to your WordPress users much like featured images on posts, the images replace the gravatars.

Easy Gravatars vs Mindutopia User Thumbnails