Media Jukebox by J.River is perhaps the best music management/player out there for Microsoft Windows. First off, it has a great interface and doesn’t hog your PC like other media players *cough* Windows Media Player *cough*. It organises your music well, makes it easy to search, includes some nifty features including album cover art, visualisations, and it can even handle the more oppresive media formats (wma I’m looking at you). Its also skinable but I think it looks pretty good out of the box, and has support for some pretty nifty plug-ins.