Fretello - Duolingo for learning how to play guitar
Band-in-a-Box - Band-in-a-Box is so easy to use!
Justin Guitar - Over 750 completely free guitar lessons!
iReal Pro - iReal Pro – Music Book and Backing Tracks is a commercial music app developed by Technimo for Android and iOS devices.
Uberchord Guitar - Get better on your guitar. Fast.
ChordMill - Music play along app for all instruments and voices