Rforge

I asked and googled this questions a lot of times. I even used time on awkward Python and even C scripts to extract the download location from the html/php code of the youtube webpage. It turns out to be WAAAY easier and I remember doing this ages ago with Netscape and some flashmovies somewhere (Youtube, even Google did not exist):

I am just quoting FRuMMaGe on Ubuntuforums

Many people have been asking how to download youtube videos, and kept being pointed to useless software which will do it for them.

Here is the easy way:

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!