Note: The developer had announced the end-of-development for Songbird.... "Unfortunately, the company has found ourselves unable to fund further business operations and as of June 28, 2013 all of Songbird’s operations and associated services will be discontinued." They go on to recommend Nightingale to those who want a free, Open-Source alternative.
Songbird is a cross-platform music player based on Mozilla code. It lets you play, manage, and discover music. You can customize its appearance with skins, and create dynamic playlists that automatically update based on criteria you set.