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!

qakcn

This plugin replaces build-in Gravatar with Tietuku Avatar Service. And replace Google fonts and scripts with USTC mirror. All for Chinese users.
这个插件用贴图库头像服务替代了WordPress内置的Gravatar，并且用中国电子科技大学的镜像替代Google字体和脚本。中国用户都懂的。

Tietuku Avatar for WordPress

Use Tietuku Avatar service to replace Gravatar. And replace Google fonts and scripts. 用贴图库头像服务替代Gravatar。并且替换Google字体和脚本。

Easy Gravatars vs Tietuku Avatar for WordPress