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.
Wikimedia
Wikipedia: Musical Mode
This site from the Wikipedia Encyclopedia provides an encyclopedia article about modes in music discusses how their names were derived, lists the major and minor modes, lists characteristics of each mode and examples of songs that you...
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.
Other
Neanderthal Flute
Site provides information on the investigation of the Neanderthal flute that was found.