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.

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.

Gravatar Signup Encouragement vs Mindutopia User Thumbnails