Auto Replay for YouTube™ per Mark Lumnert

Auto Replay for YouTube™ allows you to automatically repeat your favorite YouTube video. It also allows you to automatically start and loop at a custom time-slot.

Some Features:
a. Allows you to loop through a YouTube video.
b. Easily loop through specific a time frame(s) by setting the start and end time in seconds.
c. Turn off the auto loop from toolbar-popup UI.
d. Supports both YouTube's HTML5 and Flash players.

Please note: in order for the Auto Loop to works, the current video needs to come to an end.