Grant Mangham

Unfeature simply associates a checkbox with the featured image of a specific post, allowing you to control whether or not this image will show on the single template.

Unfeature

Simply associates a checkbox with the featured image, allowing it to be skipped on single posts.

tsaiid

Featured Image via URL is a plugin to generate the featured image of post from the image url specified. 
The image will be fetched back and saved into the Media Library with all supported sizes. 
If the featured image is already present, the plugin will do nothing.
This plugin is modified from Auto Post Thumbnail by adityamooley.
For more details, see http://tsai.it/project/wordpress-plugins/featured-image-via-url/

Featured Image via URL

Automatically generate the Featured Image from the specified image url in post.

Unfeature vs Featured Image via URL