YouTube Music Player is an extension for Mozilla Firefox that lets you use YouTube's video database as your own playlist.
All you need to do is click the extension icon, which will be displayed at the top-right corner of the screen and start adding videos. The selected videos start loading before you play, so you never have to wait to hear a song.
The program does not just allow you to drag and drop videos into videos, but you can do anything in any regular music player, do the same thing as pashing, repeating, shuffling etc.
The YouTube Music Player is a wonderful extension for those music lovers using Firefox, who can find virtually infinite hours of music.