Songbird is free and open source audio player. It works on GNU/Linux, Windows and Mac OS. It plays mutliple file formats such as MP3, AAC, Ogg Vorbis, FLAC, WMA and others. You can get cool skins for the application and also add extensions to give it more features. You can download album art, lyrics, concert ticket info, band info etc. Songbird can also be used to play media from the web. This application is comparable to iTunes and you can see some of the resemblance. Songbird is currently in active development and there are many functions that are going to be added so stay tuned.
To download Songbird go to their website.