Module Link: http://drupal.org/project/file_aliases The File Aliases module allows you to alias where files are in your system so they don't appear to be stuck in /sites/default/files.
Who: Technical and non-technical users When: So far I've thought three uses for this module 1) SEO people complaining about the path to your files (not joking) and 2) When you want to hide the fact that your site is in a multi-site installation and 3) When you have Flash requesting a file in a specific location and you can't change the Flash source. Where: In the configuration options for your file field (CCK). Why: Because getting SEO people off your back is worth installing an extra module for. I've never had a very concrete reason to run this app but I'm sure plenty of people have.
P.S. The module at this time only creates the alias, the default node link to the file still goes to /sites/default/files and I haven't been able to change it. There is a bug out there for this issue so hopefully i
