I play songs on youtube all the time, and sometimes have wanted to play an entire song in a loop. However this feature is not present directly on youtube. I just found this tool which allows you to play a youtube video in a loop: http://www.endlessyoutube.com/

You need to take the youtube video URL: http://www.youtube.com/watch?v=se9rfWucgeY

Copy the VideoID (the highlighted part in the link above) and paste it onto the box at endlessyoutube. Now you are all set to listen to the video in a repeat loop.