You could use the built-in OS X ftp command, and call it with an Applescript "do shell" or a free plug-in.
I've used an FTP utility called Fetch to do most of my FTP dirty work in OSX.
It supports applescript commands and you can use it to "record" applescript commands to later use in a filemaker shell script as noted by fmp above.
360Works also has a super-robust FTP plugin for filemaker that is cross-platform compatible.
Yes, the 360Works FTP client works great for me too. And now that installing plugins from containers is easy, it is easy to distribute to users. Actually, one of the most useful things I ever did was get the 360Works Portfolio license and then I can make use of all of their plugins. Pretty nice deal for the price!
Fetch... wow... I remember when that came out at the University of Michigan servers in the early 90's. Do you remember Gopher too? Fetch is a product that sure has lasted a long time and worked well. But for now, I personally prefer Transmit. It sure is nice on the Mac to be able to mount an FTP site and Transmit lets me do that and is a pretty unique feature among FTP clients.