If you're using Safari, there's an easy way to download YouTube videos.
- Open the page with the movie and press Command-Option-A, which shows the Activity window.
- In the Activity window, click on the arrow to show details about what is being loaded.
- You will certainly notice an element whose size is over 0.5MB (most of the time, over 5MB). Double-click on it (even if it is still loading), and Safari will download it.
- When the download is over, navigate to the file in the Finder (which will probably be called get_video), probably on your desktop.
- Rename the file to something you will recognize and add the extension .flv to its name.
- Now you can play it with VLC media player (which is probably located in your Applications folder) or with QuickTime (only if you have Perian installed).
(from Macosxhints.com via Google search)
