apt-get install youtube-dl
If that version is too old, take it directly from github (it's a python script): http://rg3.github.com/youtube-dl/
Download video:
youtube-dl 'http://www.youtube.com/watch?v=Lj-Rsl8brt8'
List formats:
youtube-dl -F 'Lj-Rsl8brt8'
Pick format:
youtube-dl -f 171 'Lj-Rsl8brt8'
Get only audio:
youtube-dl -x 7J2ic0q80mY
Dump audio:
avconv -i Lj-Rsl8brt8.mp4 out.wav
Convert to mp3:
lame -h -v -B 192 out.wav out.mp3