If you liked a particular YouTube video so much that you want to view it again an again without bothering to click on the Replay button every time, then there is a Greasemonkey script to do that for you.
Loopy for YouTube, adds a small button below the video that allows you to play the video in an infinite loop. Of course, you can turn it off!
To play a YouTube video embedded in a page, in a loop, append the video URL with the loop parameter &loop=1. Example
http://www.youtube.com/v/VIDEOID&loop=1
There are a few more parameters available that customizes the look of the player to your liking. View the complete list.
[via Google Operating System]
Create a YouTube playlist and then select 'Play all'.
http://www.ehow.com/how_2031098_watch-list-youtube.html
Find the YouTube video you want to repeat.
In the address bar, add the word "infinite" before the word "youtube". This is the easiest way to put YouTube videos on repeat. When you are finished your address bar should be changed from:
http://www.youtube.com/watch?v=dMH0bHeiRNg
To
http://www.infiniteyoutube.com/watch?v=dMH0bHeiRNg
You can also go to http://www.infiniteyoutube.com and paste in a link to the video or even search for videos.