youtube-dl - https://github.com/rg3/youtube-dl
download video:
youtube-dl -o '%(title)s.%(ext)s' X74tQvF0Mrk --restrict-filenames
download mp3:
youtube-dl -o '%(title)s.%(ext)s' X74tQvF0Mrk --restrict-filenames -x -x --audio-format "mp3"