From Flavio Tordini:
Musictube is a YouTube music player. With it you can listen to millions of songs on YouTube in a convenient way, much like using a traditional player. Musictube plays album tracks in their original order and integrates album covers, artist pictures and lyrics. Musictube can play full albums or even discographies of millions of artists present on YouTube. Besides listening to plain studio recordings, you can switch to "live mode" to listen (and watch) live performances or to "cover mode" to discover surprising new versions. Musictube has a fully editable playlist you can fill in seconds and let play for hours. You can always show the video as a thumbnail or full-window.