I've have done some research and a found a lot out there:
Internal FTP / PHP web based file transfer
This works ok, I've been using it for a while but I am finding that I spend to much time with security and cleanup. I've decided that I don't really want all kinds of doors on my internal network open to the internet. I want to let someone else, hopefully more qualified them me, handle security and maintenance.
Torrent & P2P
I'd love to use torrents or p2p but a client install is something I want to avoid for the sake of my non-computer savvy customers.
Requires an account signup in order for my customers to send files to me.
Nice, but it centers around an internal network of specific users. My user base is going to change from day to day depending on which customers I need to share files with. Box.net would break me a $15/user.
Requires a client install
Requires an account setup to upload.
All I want to require my customers to do is type or cut & paste a URL (which I supply), enter a username and password (which I supply), then click a download or upload button.
No client install, no account signup, no confusing menus, and no ungodly per/user charges. I want the files to be private per user and to expire after a specific time.
Does anyone have any ideas?