Rocksmith+ - Rocksmith builds off of the fundamentals of music games like Rock Band and Guitar Hero by offering fundamental lessons in actually playing guitar.
Spotify - Map shows when two people play same song at same time
Frets on Fire - Bonus content.
SoundCloud - Enjoy music & follow favourite artists
Guitar Hero Live - Guitar Hero Live is a Single and Multiplayer, Rhythm and Music video game created by FreeStyleGames and published by Activision.
Last.fm - The world's largest online music service. Listen online, find out more about your favourite artists, and get music recommendations, only at Last.fm