You can find a lot of alternatives for this subject as you can find any other applications in ubuntu. This the beautiful side of liinux and free os platforms. Of course there is an easy way and difficult way. First the easy one:
Step 1: Find the video you want to download and wait for it to completely finish loading.
Step 2: Minimize your browser (do NOT close it, some browsers delete the temp directory upon closing), and navigate to the directory /tmp
Step 3: Drag the flash file onto your desktop. Voila!
This definitely works with firefox, and probably most other browsers aswell. Enjoy!
And there is an another easy way to download videos from youtube in ubuntu and any other operating system; the online applications. For example Vixy.net is a site that allows visitors to download youtube videos in any format they want.
There is another online way to download youtube videos in ubuntu. Here is a simple way also: Find the video you want to download…let’s say this one here:http://youtube.com/watch?v=2BCoZiSbGtY
You don’t need to watch it or anything…then in the address bar on top of ANY browser put kiss in front of the youtube…so the address will be http://kissyoutube.com/watch?v=2BCoZiSbGtY
That will take you to a page that will give you the preview of the video(like youtube) and a lot of links …and one of them is Download Now (guess what it does )
And the hardway is to install an aplication to download a video. Here how to do it: Install youtube-dl in Ubuntu
sudo aptitude install youtube-dl
This will complete the installation.
Using youtube-dl
Usage instructions are easy. Use youtube-dl followed by a video URL or identifier.
youtube-dl Example
youtube-dl “http://www.youtube.com/watch?v=foobar”
The video will be saved to the file foobar.flv in that example. As YouTube.com videos are in Flash Video format, their extension should be flv.Check this simple guide to convert .flv files to .mpg
Enjoy videos!


there is an add-on for firefox that allow you to downdoad media stream content from internet(YouTube, lastfm, imeem etc..): http://labs.carcast.inrete.it
There is also miro, a full-fledged video downloader / viewer.
thnks