Here is another cool YouTube app that does one simple thing: It lets you watch any YouTube clip over and over again without hitting the play button. Basically, if you stumble upon a video (maybe a music video) you really like and feel like watching over and over, copy the video URL onto the provided field on TubeReplay and click the “Replay” button.
If you do not feel like copying URLs manually then you may try alternative Firefox add-on [No Longer Available]. This add-on adds an auto replay checkbox to YouTube page along with additional options to specify the exact part of the video you want replayed. Pretty cool.
Check out TubeReplay @ www.tubereplay.com