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, 320×262, 25.00 fps(r)
  Resize: 320×262
      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