RealPlayer 11 is a perfect solution for me. Much as I've generally hated Real's products, 11 is not evil, does not take over the world and your computer, and works beautifully. Install the Downloader agent when it asks you on installation, and then every time you watch a youtube video (works on most but not every flash site, but on more sites than any firefox plugin I've found) in either Safari or Firefox (don't know about other browsers) it runs and there's an unobtrusive dock icon and a window with a download link in it. If you click download it does just that, and saves it as a .flv with no quality loss as far as I can tell. And it's free.