“Tones sound, and roar and storm about me until I have set them down in notes.”
Ludwig van Beethoven

Music Theory for Advanced Students

Please check also the links for beginners; many of the links begin with the basic stuff, but advance quickly to more complex music theory.

Music Theory & History Online by Dr. Brian Blood: a comprehensive tutorial on the theory and history of music. This tutorial begins with the basic and is highly academic. Could be used by beginners, but I would not recommend it to children.
Theory on the Web by Robert J. Frank: includes fundamentals, harmony, part writing, basic chromatic harmony.
Learn Piano Chords
Harmonic Dictations: For advanced students.
Jazz Theory
Music “Stuff” Links: Many useful and interesting links. Some listed here, some broken links but also some good links...