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

Nicolas Juen

Jason Rouet

The Mediapapa Team

This plugin lets you create custom image sizes for your site. Override your theme sizes directly on the Media settings page, regenerate thumbnails, and copy <code>add_image_size</code> PHP for your theme.
<h3>ℹ️ Simple Image Sizes, a Mediapapa product</h3>
Simple Image Sizes will always remain free. It is actively maintained by me (Nicolas Juen). I’m also building <a href="https://wordpress.org/plugins/mediapapa/" rel="ugc">Mediapapa</a>, a complementary plugin focused on understanding and organizing your media library. <a href="https://www.wp-mediapapa.com/blog/simple-image-sizes-update/" rel="nofollow ugc">> More on these changes</a>
<h3>Features</h3>
<ul>
<li>Insert images in posts at the right size using your custom sizes.</li>
<li>Control whether each size appears in the “Insert media” size dropdown.</li>
<li>Regenerate thumbnails in bulk from Settings → Media, and pick which sizes to rebuild.</li>
<li>Copy generated <code>add_image_size</code> PHP for your theme when you want to keep sizes and deactivate the plugin.</li>
<li>Regenerate from the Media library: one file at a time or with list bulk actions.</li>
<li>Regenerate from a single attachment’s edit screen.</li>
<li>Approximate end time and clearer feedback while bulk regeneration runs.</li>
</ul>
<a href="https://github.com/Rahe/Simple-image-sizes" rel="nofollow ugc">Contribute on GitHub directly.</a>

Simple Image Sizes

This plugin lets you create custom image sizes for your site. Override your theme sizes directly on the Media settings page, regenerate thumbnails, an …

Gmagick vs Simple Image Sizes