Did you know that most YouTube videos can be downloaded now as high-quality mp4 (MPEG-4) files? There have been browser and URL hacks to download files previously, and several sites and Firefox extensions would allow you to plug-in an URL to a YouTube video and it would snag the file or give you an URL to download.
Now, you can do the same thing to get the higher-quality videos. Just save the following as a bookmarklet by dragging it up to your bookmarks bar in Firefox or Safari, or right-clicking and making it a Favorite in Internet Explorer:
Clicking on this bookmark creates a new link below the video you are watching that should be to the downloadable video. Instead of left-clicking, you’ll want to right-click and choose “Save As” and name it something with a .mp4 extension.
I ripped-off came upon these instructions from a blog called Google Operating System via a Lifehacker post, and there you can find more information, including a Greasemonkey script for Firefox that takes care of this whole business for you every time you visit YouTube.