Harmony.org.ukSave
By Tom Sutcliffe. Aims to help students of music theory understand the role of chord progressions in musical structures. Site includes animated demos. http://www.harmony.org.uk/
SchenkerGuideSave
An introduction to Schenkerian analysis for undergraduate music students. Includes background, working method, glossary and bibliography. http://www.schenkerguide.com/
The Musical OctaveSave
Thomas Váczy Hightower's study of musical scales. Discuses the meta-physical properties of music and the creation of musical scales from an acoustic and mathematical perspective. http://home22.inet.tele.dk/hightower/
The Tonal CentreSave
Interactive site for music composers and theorists which explains and demonstrates some of the key concepts of tonality; including chords, scales, cadences, and modulation. http://www.tonalcentre.org/