My Firefox Download Helper, or whatever the thing is called, wasn't working for me so I searched for another program or whatever to download them, something that wasn't too technical and would be easy enough even for me to understand. There is a bash script you can use (I guess, never tried it) but I did find a simple way. Here it is: Re: HowTo: Download YouTube videos (the easy way)
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 )
Your mileage may vary but it worked like a charm for me (my computer is PCLinuxOS). I am assuming it should work on any Linux box and probably even on Windows but I don't know for sure since I don't use Windows.