-x, --extract-audio        convert video files to audio-only files (requires
                           ffmpeg or avconv and ffprobe or avprobe)
--audio-format FORMAT      "best", "aac", "vorbis", "mp3", "m4a", "opus", or
                           "wav"; best by default
--audio-quality QUALITY    ffmpeg/avconv audio quality specification, insert
                           a value between 0 (better) and 9 (worse) for VBR
                           or a specific bitrate like 128K (default 5)
--recode-video FORMAT      Encode the video to another format if necessary
                           (currently supported: mp4|flv|ogg|webm)
-k, --keep-video           keeps the video file on disk after the post-
                           processing; the video is erased by default
--no-post-overwrites       do not overwrite post-processed files; the post-
                           processed files are overwritten by default

Supported Websites .. 
youtube , metacafe , dailymotion , video.google:search , photobucket , Yahoo , screen.yahoo:search , DepositFiles , facebook , blip.tv , blip.tv:user , vimeo , myvideo , ComedyCentral , Escapist , CollegeHumor (CURRENTLY BROKEN) , XVideos , soundcloud:set , soundcloud , InfoQ , mixcloud (CURRENTLY BROKEN) , stanfordoc , MTV (CURRENTLY BROKEN) , Youku , XNXX , YouJizz , Pornotube , YouPorn , plus.google , arte.tv , NBA , WorldStarHipHop , justin.tv , FunnyOrDie , Steam , ustream , RBMARadio , 8tracks , keek , TED , MySpass , Spiegel , liveleak , ARD , ZDF , Tumblr , Bandcamp , RedTube , Ina , Howcast , Vine , Flickr , Teamcoco , XHamster , Hypem , Vbox7 , Gametrailers , Statigram , Break , Vevo , Jukebox , Tudou , CSpan , Wimp

$ sudo apt-get install youtube-dl

$ brew install youtube-dl

$ sudo curl -o /usr/local/bin/youtube-dl http://youtube-dl.org/downloads/2013.07.08.1/youtube-dl
$ sudo chmod a+x /usr/local/bin/youtube-dl