For those with a Mac, I can heartily recommend Rumpus FTP server from Maxum development.
Bob K turned me on to it last year, it is an excellently designed FTP server that also supports WebDAV and HTTP, so you can point your clients to your server through a regular web browser and up/download from there, or even mount the disk (their user area) on their desktop.
Very slick, reasonably priced, secure. The best part is that if you set up a computer in the studio as the server, you clients files are there ready to be worked on, and when you're done a simple drag and drop makes them available for them to download.
It also offer remote management, statistics, logging, etc etc.
Cheers,
Thor