Older YouTube videos may not have "View in Higher Quality" links below them. But by using the following parameter in the URL, you'll be able to watch anything in higher quality (HQ)!

Examples:
Low Quality (LQ) - http://www.youtube.com/watch?v=l5DNZHR0oI8
High Quality (HQ) - http://www.youtube.com/watch?v=l5DNZHR0oI8&fmt=18

How it works:
Just add "&fmt=18" to the end of the URL at the address bar of your web-browser and press the Enter button. That's all! You get Higher Quality YouTube video.

The URL will now look like the following one

And the following YouTube video was made Higher Quality (HQ) by this hack: