Fretello - Duolingo for learning how to play guitar
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
Justin Guitar - Over 750 completely free guitar lessons!
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Uberchord Guitar - Get better on your guitar. Fast.
Scale Heaven - Browse and play any scale/chord, chord progression.