If you want to upload a file to a web site and that file's already on another site—say, an adorable dog photo from Flickr to your blog—this handy shortcut shaves a few steps off how you're probably doing it.
Rather than save the file to your desktop, then opening the upload file dialog and navigating to where you downloaded the file, you can just copy the URL to the file and paste that URL in the Select File dialog. When you hit Open, Windows takes care of downloading the file from the original server and then uploads it to the server in question without requiring the extra steps from you. It's maybe not a life-changing revelation, and it's also not new, but it was new to us, and is a nice shortcut to have in your arsenal.
[via @spolsky]