FILEZILLA [FREEMIUM OPENSOURCE] (windows, linux, mac)
FileZilla is a powerful FTP-client. It has been designed for ease of use and with support for as many features as possible, while still being fast and reliable. The program offers support for firewalls and proxy connections as well as SSL and Kerberos GSS security.
Be aware that installing FileZilla on your computer might install some bundleware/malware on your machine.
WinSCP [FREE OPENSOURCE] (windows)
WinSCP is a popular free SFTP and FTP client for Windows. Moreover, WinSCP is a powerful multi-functional tool that will improve your productivity.
WinSCP can copy files between a local and remote computer using multiple protocols: FTP, FTPS, SCP, SFTP or WebDAV. On the one hand, WinSCP offers an easy to use graphical user interface; you can choose between Windows Explorer look and tabbed twin-panel interface like Norton commander. On the other hand, advanced users can automate WinSCP functionality using .NET assembly or simple batch file scripting
Cyberduck [FREE OPENSOURCE] (windows, mac)
With an easy to use interface, connect to FTP (File Transfer Protocol), SFTP (SSH Secure File Transfer), WebDAV (Web-based Distributed Authoring and Versioning), Amazon S3, Google Storage, Windows Azure, Rackspace Cloud Files and even Google Docs.