All you have to do is to copy paste the following code on your browser's address bar with the video page opened to download the flv/mp4 on your PC.

You can play flv/mp4 file with VLC player.

Plain quality:

javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + yt.getConfig('SWF_ARGS')['video_id'] + "&l=" + yt.getConfig('SWF_ARGS')['l'] + "&sk=" + yt.getConfig('SWF_ARGS')['sk'] + '&t=' + yt.getConfig('SWF_ARGS')['t'];

High quality:

javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + yt.getConfig('SWF_ARGS')['video_id'] + "&fmt=18&l=" + yt.getConfig('SWF_ARGS')['l'] + "&sk=" + yt.getConfig('SWF_ARGS')['sk'] + '&t=' + yt.getConfig('SWF_ARGS')['t'];

HD quality:

javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + yt.getConfig('SWF_ARGS')['video_id'] + "&fmt=22&l=" + yt.getConfig('SWF_ARGS')['l'] + "&sk=" + yt.getConfig('SWF_ARGS')['sk'] + '&t=' + yt.getConfig('SWF_ARGS')['t'];