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Am told that if you append
&fmt=6 or
&fmt=18 to the end of a YouTube URL, you get to watch the video in much higher quality. Am not sure if this applies to all videos, or just the ones that already have a perfectly good "watch in high quality" button.
Here is a good example - and here is the same video
with the hack.
And here - probably a joke - are
special glasses that make DVD and HDTV look just a crappy as YouTube usually does.