Rhythmbox is an open source audio player that has taken a design cue from Apple's iTunes package and run with it. Using the gstreamer audio libraries, it supports both mp3 and ogg-vorbis playback, and provides an alternative to XMMS or mpg321 on unixy platforms. Much like iTunes, it organizes music into a central library, allowing the user to then create playlists with selections from their library. Plans for the future involve the ability to rip/burn audio cds as well as the implementation of visualization plugins. While the project is still young, it has come a long way in recent months, and aims to eventually become a part of the GNOME distribution.
Rhythmbox can be acquired using Debian's apt-get package management software, if you run the unstable distribution, and RPM packages have been made available as well. Of course, source code tarballs are also available and code can be checked out from the gnome project cvs server (anoncvs.gnome.org).