WinSCP is an open-source secure file transfer program that uses SSH and supports Secure FTP, legacy SCP, and plain-vanilla FTP.
The program offers two interfaces: a Windows-based view that displays only the remote server (you drag-and-drop files from your desktop to initiate a transfer) and an interface similar to Norton Commander that shows you both local and remote directories. Casual users may prefer the Windows view, since most of the advanced features are out of sight in drop-down lists; the comprehensive Commander-style view will likely please power users and server admins. These users will appreciate the function key shortcuts that provide quick access to common tasks.
I tried both interfaces and found them easy to use. In my basic file-transfer tests using FTP and SFTP I found WinSCP to be very quick and stable. You can perform basic file management tasks on your remote server--moving and deleting files, creating directories, and so on--and there's even a built-in text editor. If you have more advanced needs, WinSCP can likely meet them; the program is packed with features that I didn't even begin to explore. Visit Martin Prikryl's Web site and browse the extensive FAQ and About sections for details.
WinSCP is offered under the Gnu General Public License, so its source code is available on the author's site. It's completely free, but Prikryl accepts donations via PayPal on his site. Since he appears to work tirelessly improving the program, it's a good idea to contribute to the cause.