I do not know how to ask it, but I'll explain it in a different way.
You know the program called PHP Gallery http://gallery.menalto.com/ - A peace of software that you can install to a web server. And you can then manage users and the images uploaded by them.
Well, I am looking a similar installable program - but for an FTP functionality. A program where I can set up user accounts, their upload disk quatas etc. The idea is to let my customers to be able to upload all sort of files - not only images. It does not have to be FTP - a user friendly file upload interface will do just fine.
I am sure there are many such upload scripts out there but, what are the biggest well-know most popular ones?
I cant recommend any as I dont use them but what you are describing is an ftp file manager script.
may I draw your attention to http://www.webfileman.com - an online multiuser file manager.