Guitar Dashboard - Open-source interactive music theory for guitarists.
Fretastic - An interactive fretboard that lets you look up scales and chord in CAGED, 3nps and other shapes.
UltimateGuitar.com - Learn how to play your favourite songs on guitar or ukulele
Fine Tuner - Fine Tuner is a minimalistic guitar tuner that utilizes your device's built-in microphone.
MyGuitarTuner.com - Online microphone tuner for guitar, ukulele, violin and other instruments. Tune by ear or using the microphone of your device. It's fast and accurate algorithm makes it one of the best online microphone tuners available.
AudioTheory Guitars - The ultimate reference tool for guitar notes and chords.