Here's something I only recently discovered - you can now download YouTube videos directly as MP4 files. The information is here. (Just drag the "Get YouTube video" link in the middle of the page to your browser's toolbar.)
Next time you see a YouTube video you might want to keep, just click the URL and it will download a copy of the underlying video file.
Prior to this, I had been using the RealPlayer Downloader - which is fine if you like RealPlayer (I don't).
