Songbird
From Wiki:
* Cross-platform compatibility with Windows XP, Vista, Mac OS X v10.5 (x86, x86-64) and Linux (x86, x86-64). Mac OS X v10.4-10.5 (PPC) and Windows 2000 are unofficially supported by either 1.0 for PPC[3] or using the official Win32 version to run on 2000.
* Ability to play multiple audio formats, such as MP3, AAC, Ogg Vorbis, FLAC, Apple Lossless and WMA
* Ability to play Apple FairPlay-encoded audio on Windows and Mac platforms via hooks into QuickTime (authorization takes place in iTunes)[4]
* Ability to play Windows Media DRM audio on Windows platforms[4]
* A skinnable interface, via feathers
* Media files stored on pages viewed in the browser show up as playable files in Songbird
* MP3 file download
* Ability to subscribe to MP3 blogs as playlists
* User-created bookmarks
* Ability to build custom mixes
* Ability to scan the user's computer for all audio files and add them to a local library
* A configurable and collapsible graphical user interface similar to iTunes, and mini-player mode
* Keyboard shortcuts and media keyboard support
* Automatic updates
* Last.fm integration via a plugin, complete with love/hate buttons
* Insound.com and HypeMachine integration
* Microsoft MTP compatible device support
* Ability to edit and save metadata tags