Visit http://www.arrakis.es/~rggi3/youtube-dl/, read a bit (includes instructions for Windows users). Download the python script and also install ffmpeg2theora (See #2 at my Drupal Video Guide).

Then:

1. Visit your favorite YouTube video
2. Copy the URL to the clipboard
3. In command line:

   magicfab@suzy:~$ python youtube-dl.py http://www.youtube.com/watch?v=PcuF7S3r6gM
   Retrieving video webpage... done.
   Extracting video URL parameters... done.
   Video data found at http://lax-v11.lax.youtube.com/get_video?video_id=PcuF7S3r6gM
   Retrieving video data... 1897k of 1897k done.
   Video data saved to PcuF7S3r6gM.flv
   magicfab@suzy:~$ 

4. Convert it (careful, those are TWO dashes before the optimize option):

   magicfab@suzy:~$ ffmpeg2theora PcuF7S3r6gM.flv ––optimize
   Input #0, flv, from 'PcuF7S3r6gM.flv':
     Duration: 00:00:47.3, bitrate: N/A
     Stream #0.0: Audio: mp3, 22050 Hz, mono
     Stream #0.1: Video: flv, yuv420p, 320x262, 25.00 fps(r)
     Resize: 320x262
         0:00:47.33 audio: 36kbps video: 305kbps
   magicfab@suzy:~$

5. An .ogg corresponding file will be created in your current directory.
6. Profit ?

update: youtube-dl has been packaged since Edgy. Install it using the standard Synaptic package manager or through apt-get in command line, by searching for the youtube-dl package.

usage: youtube-dl [options] video_url