ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
Fretello - Duolingo for learning how to play guitar
soundrop - Soundrop is an app that allows you to create your own personalized sound track to keep you focused or relax.
Justin Guitar - Over 750 completely free guitar lessons!
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
Uberchord Guitar - Get better on your guitar. Fast.