Import Linked Images
It's frustrating to go back to a blog post and see that its images no longer exist because whatever site was hosting them is now gone. This plugin automatically downloads any external image sources and replaces them with the new local copies.
There is another plugin on the WordPress.org repository that does this, called Add Linked Images To Gallery. Unfortunately it's no longer maintained and has some very messy code. This is a first pass and cleaning up and simplifying that idea.