Simply add the word "repeat" in your address bar between "youtube" and ".com" and press Enter.
For example, turn
http://www.youtube.com/watch?v=cB7gPzorDz0
into
http://www.youtuberepeat.com/watch?v=cB7gPzorDz0
This works great when you really like a song and want to listen to it over and over or you get one of those mandatory 15sec/30sec ads on YouTube.
Enjoy! ^_^