Recently I've discovered the excellent Songbird music player. This has only just reached version 0.5, but is already showing a truly staggering amount of promise. Once it reaches version 1.0, it may even replace Amarok in my affections. I'm already using it instead of iTunes on my Vista laptop for playing music (I haven't tried it with my iPod yet). I would use it in Kubuntu, but the only package I can find for it (via Getdeb) is a bit cranky.
For the uninitiated, Songbird is to iTunes what Firefox is to Internet Explorer. It's an open source music player that supports extensions, similar to Firefox. It's written by the people that created WinAmp and is built around XULRunner, the Mozilla framework, which is what allows it to support the extensions, similar to Firefox. It can work with just about any online store that wants to cooperate with them.
To put it another way, imagine you took iTunes, ripped out the iTunes store and replaced it with Firefox, so you could browse music sites and listen to music you find, all within the same application.
It already has a staggering number of extensions, including ones that allow you to scrobble to Last.fm, which makes me think it will almost certainly be a success. You can skin it as well (these skins are known as "Feathers"). And even though it's a fair bit weightier than the likes of Amarok, it's still a lot smaller and faster than iTunes.
If you haven't tried it, download it and give it a go. It will change the way you think about enjoying music on the Internet.