It doesn't really fulfill your drag-n-drop requirement, but I've used phpXplorer (http://www.phpexplor....org/phpXplorer/www/
) and found it to be pretty useful.
I'd be interested if anyone can find a free PHP, Perl, or Python based web app that does. I don't think my provider supports Java on the server, but a client side Java applet to handle the drag-n-drop would be fine.
I came across JUpload (http://jupload.sourceforge.net
) which looks interesting from the screen shot and the description, but I was unable to get the demo applet to work here. It didn't start at all in Firefox (which may be a Firefox/Java configuration issue) and in IE6 the applet started, but file uploads failed (but that may be because the demo server may not really be capable of accepting files).
If you try it let us know how it works out for you.