Theory

Banbury School Music Department - Offers resources to assist students with music courses. Located in Banbury, Oxfordshire.
http://www.musicdepartment.org.uk/

The Basics of Reading Music - Kevin Meixner offers a free tutorial that teaches about the staff, treble clefs, bass clefs, and note names. Includes audio samples.
http://www.notationmachine.com/how_to_read_sheetmusic/readingmusic.htm

Java Music Theory - Java applets designed to help students of music improve their basic music reading skills.
http://www.emusictheory.com/

Happy Note - Offers computer games that allow kids to learn how to read music in a fun way.
http://www.happynote.com/music/learn.html

Gary Ewer's Easy Music Theory - 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.
http://musictheory.halifax.ns.ca/lessons.html

Theory Basics - Learn basic music theory principles, key signatures, note values, and clef symbols.
http://theheartstring.org/theory_basics.htm

A.D.A.M. Music School - Offers free online music lessons for beginners. Registration required.
http://www.adam2-1.com/music.html

Elements of Music Notation - Basic introduction to reading music. Learn note names, clefs, staff, and signs. Worksheets with answer keys included.
http://www.lessontutor.com/eesMusic1.html

Introduction to Reading Music - Uses simple terms to introduce the most important topics in reading music.
http://datadragon.com/education/reading/

Music Theory - Articles, online reference books, interactive exercises, and questions.
http://www.teoria.com/

Guitar Chords - Offers an interactive demonstration of various guitar chords. [Requires Java]
http://www.netrover.com/~mary3384/guitar/guitar.htm