Other
Dr. Brian Blood: Music Theory and History Online
An exhaustive resource with everything concerning music theory. From ear training to music history, this site covers it all.
PBS
Classics for Kids: Musical Dictionary
This online music dictionary offers more than just definitions. Most terms have vocalized pronunciations and audio examples, as well.
Other
Online Music Theory Reference
This site is essentially an online music theory dictionary or encyclopedia.It teaches about intervals, chords and scales. Online sound clips really enhance site.
Other
Tonalsoft: Encyclopedia of Microtonal Music Theory
Find an online encyclopedia specializing in microtonal music theory.