Cyberduck is a free open source FTP and SFTP (SSH Secure File Transfer) browser licenced under the GPL with an easy to use interface, integration with external editors and support for many Mac OS X system technologies such as Spotlight, Bonjour, the Keychain and AppleScript. I use it often to maintain the Alumnae Association web site. The Quarterly staff uses it upload large files to our FTP server for easy transfer to the magazine designer and printer. The editor no longer has to burn files to CD and deliver them or mail them. The software is being actively developed, but it automatically updates and reinstalls itself.
When I'm working from my Windows computer I use WinSCP. it too is a free open source SFTP client using SSH, which is much more secure than plain FTP.