Add Linked Images to Gallery is a WordPress plugin that I suspect has a fairly limited number of uses given the very specific nature of what it does, but if you are one of those folks who need this, it is a lifesaver.
The plugin will scan the IMG tag on new posts looking for links to graphic files that are on other sites. When it finds them, it will automagically grab the external file, add it to the WordPress Gallery for that post, and then change the link so that it references the locally stored file.
Why would you possibly want to use this? Well, I’m using it on my lifestream so that the occasional graphic that is sent along from feeds like StumbleUpon or Gdgt are stored locally so I don’t have to depend on those sites staying in business in order for the graphic to appear on my site.
The developer of the plugin uses it to automatically copy images uploaded from his iPhone to Flickr in WordPress posts he makes from there (as I said, the use cases here probably aren’t very common).
The one thing the plugin doesn’t do at the moment is allow an admin to go back and retroactively apply this methodology to existing posts, but that is a feature the plugin’s developer is working on.