I recently switched my home laptop over to Ubuntu Linux (really excited for
Ubuntu Studio) so I had to find an alternative for iTunes. Most didn't quite fulfill my needs, but I've been fairly with the cross-platform open-source media player,
Songbird. It's still in development, but they have a preview download that I'm currently using as my primary music player. It's pretty fucking buggy right now, but most of the time it works just fine.
It's based on Firefox, so first of all, you know it's going to have a big community of nerds supporting it. That also means it's easily extendable, which will result in a wealth of plugins (there's already several available including iPod sync, Last.FM, mp3 directory organizing, etc). There's a Firefox browser within the software and when you visit a page with any links to mp3s, they show up in a list on the bottom of the window, both playable and downloadable.
Though it has a ways to go, this promises to be a big deal later this year and is already very usable. Could be the next big media player.