Cyberduck

Cyberduck is a great little open source FTP and SFTP client by David V. Kocher which does exactly what it's supposed to do. Even if you're not an FTP expert, Cyberduck is a breeze to use with a familiar OS X interface. It's been made available in a huge range of languages (English, Czech, Dutch, Finnish, French, German, Italian, Japanese, Korean, Norwegian, Slovak, Spanish, Chinese (Traditional and Simplified), Russian, Swedish, Danish, Polish, Indonesian, Catalan, Welsh, Thai, Turkish and Hebrew), and there's a dashboard widget too.

Even if you don't really use FTP, you might want to download this software just to get a bright yellow duck icon in your dock.
 
» Cyberduck

 If you need a window-flavoured alternative, you could check out Blade FTP or Coffecup FTP