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.

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!

Gravatar Signup Encouragement vs Easy Gravatars