To answer your question, lftp [2] on the command line, cyberduck [3] on osx. Both of these tools are capable of connecting to sftp servers, too.
Also, sshfs (+ osxfuse for osx) [4] is definitely worth a try.
[1] https://tools.ietf.org/html/draft-ietf-secsh-filexfer-13
I've convinced several coworkers to buy it as well because it's so easy to use. My most favorite features is the ability to mount a server as a disk.
The most full-featured is by far the command line "lftp" client.
The best mac-client is by far Transmit.
On a more serious note. If you ask for GUI clients here, Filezilla is still quite good. But better don't get it from SourceForge, as the package it with malware, adware, trojans and what not. Oh man, crazy how much SF messed things up in the past decade... but different story.