http://www.digitaljournal.com/article/251142

YouTube Tests Higher Resolution Videos

Posted Mar 3, 2008 by Chris V. Thangham
Google’s YouTube is testing high-resolution videos at its site. By adding a small text to the YouTube link, you can watch high-resolution videos that are available on the site. In the future, YouTube will play those videos automatically.
As promised by Steve Chen last year, YouTube has started releasing high-resolution videos. You can see high-quality videos by adding the following “&fmt=6” to the URL of the video on Youtube.
Normally, we see the video (e.g. Hannah Montana’s video) in low resolution (320 x 240) here, and YouTube has added the higher quality video here.
So, to see a higher resolution of any video just add &fmt=6 as shown in the example below:
Regular version video: http://www.youtube.com/watch?v=QAE2-FQHkok
Higher resolution video: http://www.youtube.com/watch?v=QAE2-FQHkok&fmt=6
The audio also is upgraded in the high resolution video and the images are crisper. The memory sizes for the higher resolution videos will increase, however, it may download slower compared to regular videos. In this example, the difference between the two is 9MB for the regular and 22MB for the higher resolution video.
Since it is difficult to find which are higher resolution videos and which aren’t just add &fmt=18 in the end. This will play automatically if they are higher resolution videos if not they will play in regular mode.
If you are using Greasemonkey in Firefox, then you can add this script to play the higher resolution videos without having to add this text in the end.
Now you can enjoy higher resolution videos either in your computer or in high definition TV via Apple TV devices.