When you're on the YouTube page of the video you want, paste this into your web browser URL bar
javascript:if(document.location.href.match(/http:\/\/[a-zA-Z\.]*youtube\.com\/watch/)){document.location.href='
http://www.youtube.com/get_video?fmt='+(isHDAvailable?'22':'18')+'&video_id='+swfArgs['video_id']+'&t='+swfArgs['t']}
It downloads in .MP4, But there's a free program called MPEG Streamclip that can convert it to Quicktime and other formats.
