I've 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, to 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.
megaupload - Requires an account signup in order for my customers to send files to me.
Box.net - 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.
Dropbox - Requires a client install
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.
Does anyone have any ideas?