21 Jul 11 11:30 pm
Normally with something like that I've traditionally used a custom-developed solution. However
something that might fit your needs (but isn't free) is Egnyte, http://egnyte.com/
It's a full filesharing solution, but you can brand it with your logo and provide customers with logins to view and upload files. The most basic plan (which costs US$24.99/mo) allows 5 'power users' (which allows you to backup their machines to Egnyte, and allows them to have their own home folders) and 100 'web users' (which is what you'd give your clients), and it all scales from there.
To be honest, you'd find out better whether it'd fit your needs just by using it; it has a free 15-day trial, so I wholeheartedly recommend checking it out and seeing how it works for you.
Just to give you that URL again, http://egnyte.com