Milan Dinić

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This plugin shows a message with link to signup page of Gravatar (pre-filled with e-mail address) to commenters and/or users who don’t have gravatar.
Message can be shown to:
<ul>
<li>unregistered commenters when they leave text input field for e-mail address</li>
<li>registered commenters to whom their registered e-mail address is checked</li>
<li>unregistered commenters after they post a comment in a dialog, to whom their entered e-mail address is checked</li>
<li>registered commenters after they post a comment in a dialog, to whom their registered e-mail address is checked</li>
<li>registered users in administration notices, to whom their registered e-mail address is checked</li>
<li>registered users in admin bar, to whom their registered e-mail address is checked</li>
<li>registered users on their profile page, to whom their registered e-mail address is checked</li>
<li>users who fill registration form when they leave text input field for e-mail address</li>
</ul>
Options are fully customizable. See FAQ for more information.
This plugin is lightweight, it adds only one field in database which is deleted if you uninstall plugin using WordPress’ built-in feature for deletion of plugins. Also it will only load jQuery file to head of your page if it wasn’t already loaded by theme or other plugin(s). Checks for gravatar are done via simple AJAX. 
If you want to speed up your web site and save on bandwidth and server resources, it is recommended that you also install plugin <a href="http://jasonpenney.net/wordpress-plugins/use-google-libraries/" rel="nofollow ugc">Use Google Libraries</a> which will load jQuery file from <a href="http://code.google.com/apis/ajaxlibs/" rel="nofollow ugc">Google AJAX Libraries</a>.
In order to plugin works, it needs to be on server with PHP 5 and on WordPress 2.8 or above.
<iframe loading="lazy" class="youtube-player" width="750" height="422" src="https://www.youtube.com/embed/eIvm4rBkxPk?version=3&rel=1&showsearch=0&showinfo=1&iv_load_policy=1&fs=1&hl=en-US&autohide=2&cc_load_policy=1&wmode=transparent" allowfullscreen="true" style="border:0;" sandbox="allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox"></iframe>

Gravatar Signup Encouragement

Shows a message with link to Gravatar's signup page to commenters and/or users without gravatar.

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。

Gravatar Signup Encouragement vs Tietuku Avatar for WordPress