I just started using FoxyTunes with my Firefox browser and I kinda like it. It puts a music player controller down at the bottom right of the browser window, which fixed one thing that was bugging me recently: switching from Firefox to iTunes and back again (over and over again). So I went looking and found this.
But it also has oodles of Web 2.0 goodness: you can use whatever is playing and use Foxytunes' portal to return a page of related searches, including YouTube, LyricWiki, Last.fm, Flickr, Google, Hype Machine, Rhapsody, Amazon, and more. It's reedonk.