Teoría: Music Theory Web
Music theory reference, tutorials, exercises, and articles. This site is run by a professor at the Conservatory of Music of Puerto Rico; the site is available in both English and Spanish.
Understanding Basic Music Theory
A free online course on the fundamentals of music theory
MusicTheory.Net
Ricci Adams's MusicTheory.net has a number of free tools for learning basic music theory, including customizable ear training sessions, elementary notation, rhythm and meter, and key signatures. Matrix calculators (12-tone) and a staff paper generator are also included. A very handy website for the music theory student.
Peter Schubert Videos
Musicologist Peter Schubert has uploaded videos discussing and demonstrating live improvising and improvisation of a canon.
Musicards
Online flashcards for music theory. Includes flashcards for reading the tenor and alto clefs.
Musicca
Online exercises and learning tools in foundational music theory, music notation, and ear training.