FileZilla is an open-source file transfer protocol client (FTP). The app is available for most major platforms. If you run Windows, you can also download a FileZilla FTP server app. Since the Mac comes with an FTP server built-in, you don't really need another server. This FTP server is very light, it runs well and it has the advantage of being very customizable. It ships with a few themes and icon sets, and I am sure you can download more or create your own if you want.
FileZilla seems to have every feature that you would probably need in an FTP client. It has a quick connection bar that allows you to easily connect to servers without having to deal with site managers or anything like that. You simply need to type in the host of the server, a username and password, and a port. FileZilla is smart enough to type in Anonymous account details and to use port 21 (which is the default port for FTP servers) if you leave those fields blank.
On the preferences window, you can access the connection configuration wizard, which asks you a few questions and configures the app based on your answers. FileZilla supports active- and passive-mode connections and the SFTP protocol. It is a very complete FTP client. Make sure to go to the preferences window and see all it has to offer.