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.

Orangelab

Rickard Westerlind

Dramatically improve the quality of re-sized images by making WordPress use ImageMagick instead of standard GD image library.
Features
<ul>
<li>Preserve embedded color profile in re-sized image</li>
<li>Automatically recognize custom image sizes</li>
<li>Allow regeneration of existing images (optionally for selected image sizes only)</li>
<li>Configure image quality or use dynamically computed default value</li>
<li>Optimize different image sizes for either quality or size</li>
</ul>
Lnguages: English, French, German, Swedish, Turkish
Requires either ImageMagick binary or Imagick PHP module.
<h3>Contribute</h3>
Code repo available on https://github.com/orangelabweb/imagemagick-engine/

ImageMagick Engine

Maybe even better.  It uses ImageMagick Engine that is installed and allows me to easily regenerate all my images.

Works as advertised

The thumbnail regeneration could be a bit faster though. I have used it for longer than I can remember, at least 5 years.

Simple, functional

Improve the quality of re-sized images by replacing standard GD library with ImageMagick.

ONet Regenerate Thumbnails vs ImageMagick Engine