Cyberduck is a SFTP/FTP/WebDAV/Mosso/S3 browser for Mac OS X written in Java using the Cocoa Framework.
Well maintained, broad functionality & a nice UI too :)
I use this on my windows pc and my macbook. excellent ftp app. Most other FTP application's I've used have horrific convoluted interfaces (COUGH FILEZILLA/CUTEFTP COUGH), but cyberduck has a very intuitive and simple interface, its a pleasure to use for simple FTP tasks. I highly recommend this application to anyone looking for an ftp app that actually has a user friendly interface.
My favorite FTP client on Mac OS X. The UI is very good.