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
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]