Edit: Unfortunately, the images originally included in this post are gone, because of hosting problems in late 2009. My apologies.
First, it has none of the knotted mess of dependencies that Audacious has, which means I can compile it on simpler systems (like my CruXO) without having to weave through all the subpackages too. The interface is almost pure GTK2 so it follows your theme to the letter. (Don’t get me wrong: I like the way Audacious looks, it’s just that it’s sometimes a little out of place, no matter what I do to it.)
Best of all it can handle streaming audio too, so you can run it over the Internet for your favorite radio stations. I haven’t tried Real audio or some other proprietary formats, so I have my doubts about those — but mp3 and ogg are working great for me.
I leave it to you to investigate all the visualizers and playlist management options. I give it a big thumbs up for being light, fast, customizable and full-featured.
As a side note to Arch users, the version in extra actually disables the GTK2 interface, as well as the OpenGL visualizer. Edit that to restore those compilation flags, and get the goodies.
P.S.: Note to self … since AlsaPlayer’s configure routine senses the available “codecs,” it would be wise to have those installed before compiling AlsaPlayer. …