Lately I have been seeing a lot of websites which you can use to retrieve the flv url of a YouTube video. If you are browsing in a safari browser of apple it's actually very easy to get this url of the video file without using such a retrieval website. Because the addresses of everything which is downloaded to see an internet page is stored temporarily in the Activity monitor. Just click "window" in the upper menu of safari and choose "Activity monitor". Now a window will open, something like this:

Safari's Activity Monitor

Now just select the flv url, copy it, enter it in your browser url box and there you go. You can use this method on almost every flash video website for that matter.