Client file area systems – any ideas?

Hi. I’m looking for a great, simple to implement, customisable client file area system. A tool that would let me create client areas (groups) and upload files. Clients would also be able to upload. A typical kind of tool, but I’ve never really found a good one, which generates good clean code.
I’m asking here, even though it has nought to do with WordPress, but because I want something as good and as easy as WordPress. Surely some WordPress users must have come across something that would work.
Ta!