AAAaaa*geek*eeeeeee >.<
http://www.songbirdnest.com/ It's only in it's early days yet but already it's waaay more featured and usable than any other media player I've seen on the market - and ooooh boy does it make me get my geek on.
It's interface is created using XUL
It's playback uses VLC.
It runs on Windows, Linux and Mac.
It's released under the GNU GPL.
Whilst none of these facts by themselves mean it's inherently good, the implementation is incredible. It's a shining example of usability, with a list of _actually useful_ features longer than my arm. I'm geeking over the webpage playlist thing right now - the application contains a mozilla-based web browser and provides a list of links to 'mp3 blogs'. If you visit a page that lists media files, it provides the information in a playlist tab and lets you stream or download all the musicks ...and stores them for future reference. It's ace!
*geeks out some more*