Winamp, MediaMonkey, iTunes, Windows Media Player - the world is full of very capable applications to help manage your music collection, and you'll almost certainly have one or two of them already. It's not immediately obvious why you'd want to try yet another, then, but spend a few minutes with Songbird and the reasons soon begin to stack up.
Songbird starts by handling the basics very well. It can import an existing iTunes library, your Windows Music folder, and synchronise with just about any other external device attached to your PC, displaying the resulting music library in an iTunes-type interface. There's extensive support for creating and using playlists, and the program now has integrated CD ripping, too.
You can then add further features with a wide range of custom add-ons. Songbird comes with extensions to play protected Windows Media and QuickTime FairPlay audio files, display the lyrics of the current song, grab CD data from Gracenote, and check which of your library's artists are touring locally. And there are others available to add iPod support, tweak the interface, integrate with online music sites like Last.fm and SHOUTcast, and a whole lot more.
Support for MSC (Mass Storage Class) devices makes it easy to sync tracks and playlists to any compatible phone or player. You're able to subscribe to a podcast, or just a blog that posts MP3s, and it'll automatically download updates for you. And because Songbird is based on Mozilla's XULRunner network, it's also a very capable browser. If you want to look up some musical reference (or anything else, really) then just open a new tab, and enter the URL you need.
Songbird has a few problems, too: the interface can be sluggish, and we found it crashed a couple of times for no apparent reason. Still, it's a powerful tool, and if you're looking for a new music manager then Songbird is definitely worth close examination.
Note that this is the portable version of Songbird.