XMMS2 is cool. In my experience, its architecture does everything The Right Way, and their support tools (git, mantis, mediawiki, doxygen-generated documentation) are all modern.
The clientlib is good enough that clients (frontends) can be very lightweight. FWICS there are some good Qt and GTK based clients that would probably run fine on OS X, if you can be bothered getting them and their dependencies working.
Even then, other Qt and GTK based applications I have on scud always feel slightly out of place (or very out of place for those using X11). There were no Cocoa/Application Kit based GUIs on the old clientlist. I set out to create a Cocoa UI.
I like Winamp Classic’s UI; it is functional and compact, especially in windowshade mode. If you are used to the keyboard shortcuts (and they make a lot of sense with a QWERTY keyboard), you don’t need huge buttons.
Hence I made a client with similar minimalism, without trying to be Winamp-skin-compatible. I’m quite happy to use the CLI and other clients for managing the media library, but for something that’s sitting on my desktop all the time I wanted small, and I wanted it to fit in with OS X. It’s got Growl support too!
Muxic is a minimal user interface to XMMS2. It should be ready for a release soon, meanwhile you can browse the Muxic source.