Andrew Furmanczyk from howtoplaypiano.ca has created a great free series of music theory lessons: “This is a full, indepth music theory course, completely for free. Designed to start from the very basics of music theory and continue forward. There is homework assigned at the end of each lesson so you can follow along and learn music theory for yourself! I also try to make it as fun and upbeat as possible so that you won't fall asleep or get board!” Visit his youtube channel and his website to see extra info and course outlines.
- teoría - by José Rodríguez Alvira - Conservatory of Music of Puerto Rico. Comprehensive site with tutorials, exercises and reference helpfully illustrated with interactive flash guides. All in small easily digested chunks. You'll learn reading music, intervals, scales, chords and harmonic functions. This web site has ben dedicated to the study of music theory is available to the Internet community free of charge since January 1997. Highly recommended!
- Ricci Adams' Musictheory.net. Free music theory lessons, quizzes, trainers, and utilities.