Theory

A.D.A.M. Music School :

[ Kids/Teens ] - Offers free online music lessons for beginners. Registration required.
Banbury School Music Department :

[ Teens/Mature Teens ] - Offers resources to assist students with music courses. Located in Banbury, Oxfordshire.
Elements of Music Notation :

[ Teens/Mature Teens ] - Basic introduction to reading music. Learn note names, clefs, staff, and signs. Worksheets with answer keys included.
Gary Ewer's Easy Music Theory :

[ Teens/Mature Teens ] - Contains 26 free lessons that teach you the beginnings of music theory. Learn about the staff and clefs, sharps and flats, scales, key signatures, and triads.
Guitar Chords :

[ Teens/Mature Teens ] - Offers an interactive demonstration of various guitar chords. [Requires Java]
Happy Note :

[ Kids/Teens ] - Offers computer games that allow kids to learn how to read music in a fun way.
Introduction to Reading Music :

[ Kids/Teens/Mature Teens ] - Uses simple terms to introduce the most important topics in reading music.
Java Music Theory :

[ Teens/Mature Teens ] - Java applets designed to help students of music improve their basic music reading skills.
Music Theory :

[ Mature Teens ] - Articles, online reference books, interactive exercises, and questions.
Robin Maynes: Quizzes :

[ Teens ] - Features online quizzes including dynamics, key signatures, note recognition and symbol identification.
Theory Basics :

[ Teens/Mature Teens ] - Learn basic music theory principles, key signatures, note values, and clef symbols.