Transmit (on the Mac) and Cyberduck (for Mac & PC), not that I use I PC, but it's useful having something I can recommend to beleaguered Windows clients who seem to prefer making life hard for themselves instead of using a real operating system.
Ubuntu, 'real' as it is, doesn't have a decent (i.e nice) FTP client, yes, there's Filezilla, but for me it's just not pleasant, however thanks to the range of addons available for Firefox we have a really nice alternative in FireFTP.
Firefox (for me) is edging ahead of Chrome to become my default browser and the FireFTP addon is yet another area of cross platform cohesion added into the mix, in this instance providing in-browser functionality to simplify server admin and related file transfer/uploads etc.
Offering direct install from the Add-Ons website FireFTP offers both FTP and SFTP connectivity with keyfile access if required (i.e for AWS EC2 instances). It's simple and clean to use and as a seamless Firefox component you don't have to manage an extra utility.
Fig:2 - FireFTP main connection window.