I've been using this for a month or two:

http://userscripts.org/scripts/show/34765

Works for more than just youtube also. Maybe I'm lazy but it's much 
easier than going to a shell and using yt or youtube-dl.

Matthias Kilian wrote:
quoted text
> On Mon, Mar 23, 2009 at 03:39:41PM -0700, patrick keshishian wrote:
>   
>> There are methods of fetching just the video off youtube if that's all
>> you want. I think I've even seen at least two scripts in ports that
>> just do that (www/youtube-dl is one and the other I can't recall its
>> names off top of my head).
>>     
>
> net/yt