mdiamondstone

authorGrid is a sidebar widget that displays the avatar of any author on your blog with at least one published post in grid form. You 
can set how large the avatar should be, how many columns (or rows) you’d like in the grid, and the display title of the widget.

Author Grid

Sidebar widget that displays the avatar of all of the authors on your blog in grid form.

DanielAGW

Dev49.net

WP First Letter Avatar sets custom avatars for users without Gravatar. The avatar will be a first letter of the user’s name. You can also configure the plugin to use any other letter to set custom avatar.
WP First Letter Avatar includes a set of beautiful, colorful letter avatars in many sizes. Optimal size will be chosen by the plugin in order to display high quality avatar and not download, for example, big 512px avatars when only 48px is needed… PSD template for avatar is also included.
You can also create your own avatar set by creating new directory next to ‘default’ folder and following the naming convention from ‘default’.
By default, custom avatar will be set only to users without Gravatars, but you can change that in settings and not use Gravatar at all.
WP First Letter Avatar helps you bring more colors into your blog. Plus, your readers will be more willing to comment on your posts, since they can actually relate to these avatars much better than to Mystery Person.
All images were compressed using the fantastic <a href="https://tinypng.com/" rel="nofollow ugc">TinyPNG</a>, so avatars are incredibly light and ultra-high quality.
WP First Letter Avatar is also available <a href="https://github.com/Dev49net/wp-first-letter-avatar" rel="nofollow ugc">on GitHub</a>.
<h4>Compatibility with other plugins</h4>
WP First Letter Avatar is fully compatible with <a href="https://bbpress.org/" rel="nofollow ugc">bbPress</a> and <a href="http://www.gvectors.com/wpdiscuz/" rel="nofollow ugc">wpDiscuz</a>. For <a href="https://buddypress.org/" rel="nofollow ugc">BuddyPress</a> compatibility please use my other plugin – <a href="https://wordpress.org/plugins/buddypress-first-letter-avatar/" rel="ugc">BuddyPress First Letter Avatar</a>.
<h4>Requirements</h4>
WP First Letter Avatar requires at least PHP 5.4. It does not work properly on PHP 5.3.x and earlier.

WP First Letter Avatar

cool plugin

Set custom avatars for users with no Gravatar. The avatar will be the first (or any other) letter of user's name on a colorful background.

Author Grid vs WP First Letter Avatar