Windows/Linux/OSX
SongBird is the mediaplayer that I would be using if I hadn't found J River Media Jukebox. In fact, if I had been just a few months late finding it, I might have stopped looking for media players altogether.
SongBird is a unique media player based on the firefox codebase. It's biggest claim to fame is the inbuilt web-browser and related functions it includes. It utilises the mozilla model of add-ons and skins which can be downloaded from within the player itself. These include everything from ad-blockers to lyrics downloaders. It can also connect to a variety of free (legal) music sites and allows you to download files.
This is the key feature that sets SongBird apart from other media players, and an advantage it holds even over Media Jukebox. If you visit a music blog in SongBird, a bar popups along the bottom listing all the media files that this site directly links to. Think of it as a pre-emptive version of firefox download manager. It prompts you to show you what you can download, letting you visit your fave music sites and pull down the new content with the greatest of ease. This could be a very useful feature, and it's something I'd love to be using regularly, but alas I'm fairly static in my music tastes these days. The more adventurous amongst you will like it.
SongBird is in beta (version 0.7) and the design has been overhauled from the last iteration. It's now much cleaner and clearer than before, and models itself on a sort of industrial version of iTunes. It's also a boatload faster, which is very important as one of the big drawbacks of the early versions was the sluggish feel to things and the tendency to crash.
It's not perfect, in that it lacks the custom views of JukeBox, it doesn't (to my knowledge) sync to any devices, and it's got no equaliser of any kind, although one is apparently in the pipeline. Having said that, it's a lot better than most of the media players out there, and it's still only in beta. Its also available to linux and OSX users, which Jukebox is not. This fact pushes it in to that growing mass of essential apps which are available cross platform.
If you're on linux or OSx, if for some reason you didn't like Media Jukebox, or if you're just curious, check out SongBird 0.7