This is a bookmarklet that I wish I had written. But Michael Wheeler has beat me to it:
For those who find themselves wanting to pop a YouTube video out into its own window, this is the tool for you. Using the bookmarklet below, you can pop a YouTube video out into a resizable window so you can watch it while working on something else.
What’s more, this bookmarklet will automatically switch the YouTube player to HD playback.
I’ve been using this for a few weeks and I now consider this indispensable.