Officially Downlaod YouTube Videos
Now you guys can officially download YouTube videos. You don’t have to reach third party YouTube downloading software or web sites. Below the video you’ll see a download button to download the video.
Some videos are not available for download, you must purchase them to view it offline 
When you go to such videos you’ll be redirected to Google Checkout page. Also YouTube has offered a new feature “Purchased Video” to track down the purchased videos.
Currently YouTube offering downloads in MP4 format. Hope soon they’ll start to offer in different formats too.
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A while ago I create a Greasemonkey userscript to download youtube videos – guess I don’t have to maintain it any longer.
@Binny
he.. he..!
No worries, still there are videos you need to pay and download. We can use your script to download those videos