Cyberduck, like Fetch, is an example of great software that Just Works. This frequently updated app has the added bonus of being open source and free, too (although the developer welcomes donations).
Bookmarks are handled with a simple drawer; drag files on to any one of them to start an upload. Multiple connections are easy to handle by simply opening a new browser window. There are no tabs, there's no image editing support, and there's only two possible views of your files (outline and list--no icon or column views), but none of that is really a big deal, because Cyberduck is a great little client.
Cyberduck is AppleScriptable, and a selection of scripts is available for download.
While several of the other apps under discussion in this article support synchronization, Cyberduck's implementation of it is particularly nice. Files can be mirrored in any direction or properly synced--and you can check a preview of any file you like before the process starts, just to be sure you won't be losing any data.
There's also Growl support, a Quicksilver module, and a colorful Dashboard widget for quick uploads on the hoof. All in all, plenty of features and nicely implemented.
Fugu offers something a little different, partly because it's exactly an FTP client like all the others, and partly because of a different approach to interface.
Fugu displays the two-pane approach, showing you both your local files on the left and those on the remote server on the right. It's simple enough to get your head around the Toolbar controls.
But most importantly, Fugu does not do FTP. At least, not plain old FTP. It's more accurately described on the product website as "a graphical front end to the command-line Secure File Transfer application." You could open Terminal and use SFTP from there; Fugu just offers a simpler way of running the same software.
It also allows you to perform Secure Copy transfers (connect just as you would to use SFTP, and additionally specify file(s) to copy).
And there's one more trick up its sleeve: with Fugu, you can quickly create an SSH tunnel (to a server that supports it, where you have an account), and subsequently transfer files over plain old FTP through that tunnel. You'd still have to use a separate FTP client, though.
Since SFTP is increasingly a standard offering from reputable web hosts, that might sound like an overcomplicated way of doing things, but it might be useful for some.
One neat touch is the addition of support for directory uploading, something not natively available in SFTP. There's still no support for directory downloading, though, which Fugu will try and overcome by using SCP instead.
Fugu feels a little slower than some of the other clients, but unless your every working moment depends on super-fast FTP connections, that's not going to be a problem for anyone.
It's free, open source, and updates are regular. The recent 1.2 release was a Universal Binary, confirming Fugu's future on the Intel architecture. If your FTP activity is only ever going to need connections to SFTP-only servers, Fugu is an excellent choice of client.