Theme Egg

Generate Thumbnail allows you to generate all thumbnails on your site. There are already some plugins available for this, but they have one thing in common: All thumbnails are regenerate in a single step. This works fine when you don’t have that many photos on your site. When you have a lot of full-size photos, the script on the server side takes a long time to run. Unfortunately the time a script is allowed to run is limited, which sets an upper limit to the number of thumbnails you can regenerate. This number depends on the server configuration and the computing power your server has available. When you get over this limit, you won’t be able to generate your thumbnails.
Why would you want to generate your thumbnails? WordPress allows you to change the size of thumbnails. This way, you can make the size of thumbnails fit the design of your website. When you change the size to fit for a new theme, all future photos you are going to upload will have this new size. Your old thumbnails won’t be resized. That’s where this plugin comes into action. After changing the image sizes, you can generate all thumbnails. But instead of telling the server to recreate all thumbnails at once, they are generate one after another. Generateing thumbnails for one photo won’t take all too long, so you won’t run into any script timeouts. Note that you still have to wait until all thumbnails have been rebuilt. If you close the page before the task is completed, you have to start all over again.
You can also select the thumbnail sizes you want to generate, so that you don’t need to recreate all images if you’ve just changed one thumbnail-size. You can also choose to only generate post thumbnails (featured images).
This plugin requires JavaScript to be enabled.

Generate Thumbnail

Generate Thumbnail allows you to generate all thumbnails at once without script timeouts on your server.

Jordy Meow

Displays the metadata information in the Media Library. Regenerates the metadata and thumbnails even if they exist. This plugin is really useful when the metadata is broken. This is an extremely light plugin.
I am using this tool for debugging many installs and decided to make it available here. Try it, discuss it. Remarks are very welcome 🙂

Media Meta & Force Regenerate

No review for it yet, and probably need to be tried and reviewed by more. It does things Regenerate Thumbnails do not, it actually force regenerate the thumbnails, the metadata associated with it, and also visually display the content of this meta.

This plugin works

Displays the metadata information in the Media Library. Regenerates the metadata and thumbnails even if they exist.

Generate Thumbnail vs Media Meta & Force Regenerate