Looking for a solid FTP client without having to drop a lot of cash can be a real chore. You might find yourself going from program to program only to discover that you need a credit card to unlock that one feature you absolutely need. In addition to that, file transfer clients are often complex and confusing, especially to someone unfamiliar with FTP.
For both Windows and Mac, Cyberduck is an open source solution that works very well and includes support for WebDAV, Amazon S3, Google Storage, Windows Azure, Rackspace Cloud Files, and even Google Docs in addition to standard FTP and SFTP protocols.
Unlike many open source programs, the interface is far from clunky and can actually be pretty intuitive. All you need to do is select what kind of server you’re connecting to, enter your login and password and you’re done. Everything else is pretty much drag and drop.
If you favor a program that gives you tons of features with a more advanced interface, FileZilla is also a great open source alternative as well.