Just fire up http://antimatter15.github.com/player/player.html in your HTML5-compatible browser and browse to the topmost folder in your music library. The app will quickly build an index of all your tunes and let you start listening right inside your Web browser. Click on the filter library text, and you can enter a search string -- results load as you type.
There's a volume control, shuffle mode, play/pause control, and you can click and drag to skip forward or rewind during playback. As OMG! Ubuntu points out, you can even save the app to your hard drive and run it offline, which is pretty darn cool.
Not all browsers are equal when it comes to HTML5 implementation, of course. We found that Chrome worked the best, and Firefox was OK. It's also worth noting that this music player comes from the same developer that created one of our favorite restartless Firefox 4 add-ons, drag2up.