The instructions on 3ouTube.com sound easy, but they’re harder than they need to be. To use the site manually you have to change the ‘y’ in youtube to a ‘3’. But why do it yourself when the browser can do it for you?
Here’s the bookmarklet: Download YouTube Video.
Drag the above link to your bookmarks toolbar and when you’re on the youTube video page you want to download, click it. (In Internet Explorer you’ll have to right-click the link to bookmark it).