Music Theory
Music theory.net: Diatonic Triads
Descriptions of diatonic triads: major, natural minor, and harmonic minor. Provides animated illustrations, and a printable chart at the end of the lesson.
Other
The Tonal Center/diatonic Scales
A comprehensive summary of basic music theory. Commentary is thorough yet practical. Information on tonality, chords, scales, cadences, etc. This pages is on diatonic scales.