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. If you're also loading other sites, you'll see a list of them: scroll until you find the YouTube page and 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) and add the extension .flv to its name. Now you can play it with VLC or with QuickTime (only if you have Perian installed).

[robg adds: We've covered the use of the Activity window before, but with the popularity of YouTube, I felt it was worth pointing out this one specialized use.]