Marko Heijnen

CodeKitchen

DH-Shredder

In WordPress 3.5 all of the GD functionality has been abstracted out and with that power Imagick has been including to WordPress 3.5 too. Gmagick needs to wait a little bit more but with this plugin you can already use it.
If you want to know a little bit more about the changes you can look at: 
* https://make.wordpress.org/core/2012/12/06/wp_image_editor-is-incoming/ 
* http://markoheijnen.com/wordpress-new-image-manipulation/ 
* http://www.slideshare.net/markoheijnen/image-manipulation-in-wordpress-35

Gmagick

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.

Gmagick vs ImageMagick Engine