I've recently been doing some interesting work with the WebFM module.
WebFM is an AJAX file browser that can be used to manage files via the administration interface of your Drupal powered site. The file browser allows you to upload files, create and rename folders, rename existing files and - almost most importantly - allows you to re-organize your files. All these features are available via a drag and drop and 'right click' context menu interface. And of course, WebFM gives you the ability to attach any file to any number of Drupal nodes. And as an added bonus, if you move or rename a file, none of your attachments will break.
I was originally involved in the development of WebFM, but its really been Rob Milne that had done the majority of work on this great tool. Recently however, I've jumped back into its development as I've had to extend the functionality of WebFM to do a few more interesting things. As a result a few new developer features are available in WebFM that weren't there before. One is the ability for developers to add their own 'right click' context menus and define their own AJAX handlers for them. This is made possible by a new WebFM hook: extend_js.