Simply put, Filefield Sources is one of those "I've got to have this on every Drupal site I use" modules. Written by Lullabot's own Nate "quicksketch" Haug, Filefield Sources adds several options to reference existing files from the file field interface provided by CCK in Drupal 6 and by core Drupal 7.
You know that process of finding the image on one site, then downloading it to your desktop, then uploading it to your Drupal site? Sucks, doesn't it? Filefield Sources allows you to simply enter the URL of an image anywhere on the Internet and it'll suck it into your files directory. Presto!
Here are the options you'll get for your file fields once you've installed Filefield Sources:
2. File browser
3. Reference existing
4. Remote URL
Filefield Sources has recently been ported to Drupal 7, but is also available in Drupal 6, as is the IMCE Browser. While the Drupal 6 version of Filefield Source requires CCK 2.6 or higher as well as the Filefield module, these are both already built into Drupal 7 and so no other dependencies are required for the Drupal 7 version.
Okay, so what are you doing still reading this? Go get this module and build a kickass Drupal site!