For Mac OS X Cyberduck is simply one of the best ftp clients around.
Supporting both standard ftp and secure ftp (sftp) - Cyberduck is a versatile and easy to use ftp client.
Additionally Cyberduck support WebDAV, typically used for web development - and Amazon's S3.
Its great usability is powered by its file browser that makes it a breeze to transfer files from remote servers.
Also its impressive localization support is part of what makes this product great - more than 20 different languages are supported.
It features an easy to use interface that allows to toggle between bookmarks and the browser outline view. For editing files, it provides a seamless integration with several external editors. Many OS X core system technologies such as Spotlight, Bonjour, QuickLook and the Keychain are supported.