Regenerate Thumbnails: this plugin is a time-saver. If you have switched to a new theme and have stretched images, this plugin can help.
Video Thumbnails: simplifies the process of showing video thumbnails in your WordPress template. It supports YouTube, Vimeo, Vine, and other popular video services.
Multiple Post Thumbnails: want to add multiple post thumbnails to a post type? This plugin takes care of that for you.
AJAX Thumbnail Rebuild: another plugin that helps you rebuild all thumbnails on your website. It rebuilds your thumbnails gradually so your server can handle it.
Easy Add Thumbnail: a simple plugin that automatically sets the featured image to the first uploaded image in each post. Very Useful if you forgot to set featured images for your old posts.
There are many other plugins that can help you do all kinds of neat things with your featured images. The above plugins handle the most common featured image problems WordPress webmasters face regularly.