First open www.youtube.com, and select a video.
For example: See the picture, the red underlined is the ID of the video.
Now open the site: http://youtube.navi-gate.org
Now copy and paste above video ID in the search box, and press ID search.
The result may be displayed in any different language, but don't panic, just click on download(the red encircled in picture above) and save this video on your hard disk. Now use any video converter to convert it in any other common format.