Here is a simple trick to download youtube videos in any quality instantly by running a Java Script.
Download Regular Quality Youtube Video Direct Links [RQ]
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'];
Download High Quality Youtube Video Direct Links [HQ]
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'];
Download High Definition Youtube Videos Direct Links [HD]
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'];
How to use the script or where to paste this java script?
Open any Youtube Video which you want to download.
Just copy the whole script (according to Quality Needed) and paste it on the address bar of the browser where the video is open.
Now Press Enter Key and Download of that yotube video will start instantly.

Note: If you watch many videos and want to download each of those videos, than you don’t need to always copy the above script. Just Drag them to your Bookmarks Toolbar or right click on the link and save as Bookmark.