On YouTube some videos have now a link below them for watching the same video in higher quality. With clicking the link, the videos reloads with the same dimension in higher quality. It appears that this feature is retroactive, appearing on videos that were uploaded a while ago and did not have this feature originally.
Some of high quality videos on YouTube are usually considered two separate entities. Even though YouTube has a bad reputation for its low-quality videos, it is the world’s most popular video streaming and video sharing website. Probably the reason for the low-quality videos on YouTube is its architecture. First the uploaded videos are converted to flash and then their FLV files are streamed through players which produces the resulting low quality video effect.
Google OS recently discovered a trick that indicates that Google might be secretly working on improving the quality of some YouTube videos.
The trick is to append the special parameter -> "&fmt=18" or "&fmt=6" at the end of URL of the YouTube video you are watching. This will make YouTube download a high-quality video version of that particular video. The high quality video version will of course be bigger in size and therefore will take more time to load. It shall be encoded at 480×360 with an MP4 format. This trick will not work on all videos but for some it works like a charm.
As an example, click on this link to watch a video.
Now with putting the first parameter at the end of the URL you will get the same video in higher quality.
You will definitely feel the difference between low-quality and higher quality of the video.
To find more high quality videos on YouTube you can just type "High Quality" on the search bar. The other way to find them is using Google as teleporting it with navigational search box. You also can set up your YouTube Account as you want. Log in to your account and choose your the default setting for viewing videos you want(as below).
Furthermore there is a Greasemonkey script to search high quality versions of YouTube videos.