&fmt=18
at the end of the address and refresh the page, it'll play the video in high def format and provide a small link below the video to return to normal video playback mode. Here's a horrible example:
Normal Quality:
http://www.youtube.com/watch?v=HuTCSVIevWc
versus:
High Quality:
http://www.youtube.com/watch?v=HuTCSVIevWc&fmt=18
The second video will take a little longer to load, but you'll easily notice the difference in sound and video quality. Try it out when you find music or videos that don't offer the high quality playback mode. It may not always work for the video, but it's usually worth a shot for the audio.
[edit]
Better example:
Normal Quality:
http://www.youtube.com/watch?v=H6JcZv3wPuU
High Quality:
http://www.youtube.com/watch?v=H6JcZv3wPuU&fmt=18
