Website: http://www.gnome.org/projects/rhythmbox/
I found this application very good recently. I have used it a few years before but it was ... functionless before, and now it is good enough for daily use!
The interface is clean, but it doesn't mean it has limited functions, I would describe it as "iTunes for Linux", not exactly iTunes but it can handle mp3, aac and other gstreamer supported media files.
I can also listen to podcast (important!) in rhythmbox, although the interface is not as polished as iTunes.
The online radio function is not necessary for me, as the channels there are mainly for US people.
It also supports iPod, so I can use the music library with my iPod, but I'm not decide to use it to handle my iPod yet, maybe later.
