I've used RBrowser and liked it. But currently, I use Transmit from Panic, which is a popular alternative:
There is a scathing review of some of the current crop of FTP clients here:
His conclusion is that Transmit (or version 2 of it: it's now gone to 3) is the best of the bunch, and even so not as good as it could be.
Actually, I think the Mac world could do with a little more of this reviewer's "curmudgeonry". I can't think how many times I have seen people suggesting that one should use Onyx. It is a "rainmaker" - a bloated AppleScript wrapper for stuff that Unix does anyway. He takes it apart very ably:
Other utilities I like are Rixstep's own ACP:
And it is probably worth having Pacifist: