Open Music Theory
Scales, Key Signatures, Intervals, Chords
Music Theory for the 21st-c. Classroom
Basic concepts, key signatures, rhythm, intervals, chords
John Ito’s Music Theory Fundamentals
Basic concepts, key signatures, rhythm, intervals, chords and more
musictheory.net
Online exercises in Identification of note identification, key signatures, intervals, scales, chords, note construction
musictheory.net Lessons for iOS
Guitar Junky music fundamentals
Teoria (in English and Spanish)
Teoria theory online tutorials
Teoria theory online exercises
Applications
Picardy (free demos)
UTheory (free demos)
Artusi (online demos)