The first way :
select the video you want to download, play the video until it is finished, do not close your browser. Search / tmp in the Filesystem. There is a file extension. flash and then copy the files. flash. After that play video on your favorite video player.

The second way :
install youtube-dl application with the following syntax:

sudo apt-get install youtube-dl

then copy the url of the video that you select in syntak following :

youtube-dl [url-video-in-youtube]

Good luck !!!

source : http://opensource.telkomspeedy.com/wiki/index.php/Cara_mudah_download_video_dari_youtube_di_Ubuntu