[Osiris]

Youtube claims that private videos can only be seen by your account and by those accounts that you shared it with. This is however not true at all, there is an easy way to download private videos from Youtube without any problems at all. Let me first explain how a private video looks like and how you are going to find them.
A private video displays a red message when you open it in Youtube reading “This is a private video. If you have been sent this video, please make sure you accept the sender’s friend request.” Take a look at private video for an example. You can search Google for the phrase “This is a private movie” to find more private movies, some really interesting ones if you ask me..
The trick to download those private movies is simple. We use a cache to access those videos. The general url of the cache is cache.googlevideo.com/get_video?video_id= and all you need to do is to add the video id of the youtube video behind it.
You do get a download prompt afterwards and can download the video to your hard drive. This is probably the only flaw in this method, there is no way to view the video on site. Should not be a big problem though.
Ryan from Cybernet News created a nice bookmarklet that simply prompts for an Youtube id which means that you do not have to copy and paste the cache url all the time. Right-click the bookmarklet and save it to your favorites, then click on it whenever you want to download a private video.
bookmarklet
via: Cybernet News