All you need to do is to book mark the below javascript in your favourite browser and while viewing any youtube video can click on that bookmark and that's it it would download the video to your default download location.
javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + swfArgs['video_id']+ "&l=" + swfArgs['l'] + "&sk=" + swfArgs['sk'] + '&fmt_map' + swfArgs['fmt_map'] + '&t=' + swfArgs['t'];