Konrád Koller

This plugin helps you to fix any missing thumbnail issue in a fairly short time. The only thing you need to do is just few simple click and the plugin does the rest. It can be really helpful if you have thousands of images in your database.
You can regenerate thumbnails for each single image in your database or you can simply select those you really want to update. If you choose a post or page the plugin will regenerate thumbnails for each related images. You can choose from the following methods:
<ul>
<li>Regenerate thumbnails for all images in the database.</li>
<li>Bulk action in post liste: You can choose which one to update by checking the checkboxes.</li>
<li>Media library items: Move your mouse over the chosen image(s) and click on “Regenerate thumbnails” link.</li>
<li>Post editor: click on “Regenerate thumbnails” button which is next to “Add media” button. (Off by default)</li>
</ul>
Furthermore you can add your own thumbnails sizes, change default JPG compression rate and mass regenerate thumbnails. The best thing about this plugin is that you can use it with custom post types as well.

ONet Regenerate Thumbnails

Doesnt have 4.4.1 bulk generation support. But you can generate thumbnails one by one.



Doesnt have 4.4.1 support completely

Fix missing thumbnails in no time with few easy clicks. The plugin does the hard work.

thaikolja

Thumbnails Cleaner helps you keep your WordPress installation tidy by removing and regenerating thumbnails.
You can control all functions through the plugins’ main page found under “Tools”. You can:
<ul>
<li>Analyze your uploads directory, giving you an overview how many original files and thumbnails there are.</li>
<li>Clean thumbnails which means deleting them while keeping the original files.</li>
<li>Restore thumbnails to start out with a clean uploads directory. This function requires the plugin <a href="https://wordpress.org/plugins/regenerate-thumbnails/" rel="ugc">Regenerate Thumbnails</a>.</li>
<li>Create backups of your uploads directory to keep your files safe in the unlikely event that there is a problem.</li>
<li>Restore backups in case you have lost a file that was not supposed to be deleted.</li>
</ul>
Please see the <a href="https://www.koljanolte.com/wordpress/plugins/thumbnail-cleaner/documentation/" rel="nofollow ugc">official website</a> for a full 
documentation.

Thumbnail Cleaner

Cleans up your WordPress installation by removing and regenerating thumbnails.

ONet Regenerate Thumbnails vs Thumbnail Cleaner