If you have a YouTube video you covet, you can save it to your hard drive using the videodownloader Firefox extension, or using other websites such as KeepVid. For sites that do not work this way, you can grab the file from Safari's "Activities" window. Here's how:
1. Load the page with the video and let the video start playing.
2. Select Activity from the Window menu.
3. In the list of all the URLs referenced in the current page, you will see one file that is indicated as being larger than the rest. Press and hold Option, then double-click on this listing to download the file to your Downloads folder.
Now you can use FFMPEGX or the like to convert the FLV to another format, or, extract its audio.
Have fun but don't fill up your boot drive, kids. :-)