Cyberduck is the free, open source FTP/SFTP client for Mac OS X that I've been using for about a year now. It works quite well, is full-featured and free (I said that already, right?). You can also get a Dashboard widget, Growl notification and Quicksilver integration for Cyberduck.
Version 2.6 is now available, and changes include:
- Turkish Localization
- Automatic software update
- Dashboard Widget included
- Reintroduced transcript drawer in browser window
- Transcript drawer added to transfer window
- Option to use Network Diagnostics upon failed connections