This is much better, especially if you use iTunes and/or an iPod, because you can play the files using the program and add them to the iPod. It also doesn't required downloading or installing anything
http://googlesystem.blogspot.com/200...mp4-files.html
All you have to do is go to that page, and bookmark the link. Then, when you're on a Youtube page, you can click on the bookmark and it will automatically start downloading the file.