Theory

Skytopia: Music and Art Aesthetics - Author's overview of how every piece of music, every sound, and every picture can be rated on its own merits outside (as well as inside) human opinion.
http://www.skytopia.com/project/rating.html

Interactive Circle of Fifths - A free music theory tool designed to help musicians interpret chord progressions, easily transpose music to a different key, compose new music, and understand key signatures, scales, and modes.
http://randscullard.com/CircleOfFifths/

Fugue No. 17: A-Flat Major - Introduction to the essential concepts of Schenkerian analysis applied to the Ab Major fugue of Bach's Well-Tempered Clavier, Book I
http://jan.ucc.nau.edu/~tas3/wtc/i17.html

Music Theory Help Site - Intermediate music theory help covering basics through beginning formal analysis and counterpoint.
http://www.wmich.edu/mus-history/TheoryHelp/TheoryHelp.html

Rhythm Exercises - Notation for drills, rudiments and etudes for practicing rhythm.
http://www.izap.com/~pinnacle/rhythms/

8-Tone Quarto-Modes Concept - Information with mp3 files, an introduction to the "diminished-major" and its applications.
http://www.jazcraft.net/8tone.html

Eric Weisstein's Treasure Trove of Music - A reference resource on music theory, covering in brief a vast array of topics.
http://www.ericweisstein.com/encyclopedias/music/

Good Ear - Online ear training site.
http://www.good-ear.com/

Music Acoustics - The science of music. Explanations of how musical instruments work via waves and frequency modulation.
http://www.phys.unsw.edu.au/music/

The Tonal Centre - Interactive site for music composers and theorists which explains and demonstrates some of the key concepts of tonality; including chords, scales, cadences, and modulation.
http://www.tonalcentre.org/

Society for Music Theory - Includes a database of journal article from the SMT Journal.
http://www.societymusictheory.org/

Tonality Guide - Fundamentals of tonality and music theory created as an online teaching tool with written and aural examples.
http://www.tonalityguide.com/

Modes and Scales in Indian Music - A historical look at concepts of Indian scales and modes comparing North and South Indian approaches.
http://chandrakantha.com/articles/scales.html

Simplified Music Chord Theory - Explains scales and building chords from them.
http://www.rpsoft2000.com/rps_chordtheory.htm

WholeArts - Music Conservatory - Introductory dialogue for courses in music theory and composition.
http://www.wholarts.com/music/ed/intro.htm

Harmony.org.uk - By Tom Sutcliffe. Aims to help students of music theory understand the role of chord progressions in musical structures. Site includes animated demos.
http://www.harmony.org.uk/

Music Worksheets - Downloadable worksheets for elementary and middle school students.
http://www.musicatschool.co.uk/worksheets.htm

Chaos Music Paper I: Aesthetic Evaluation - Essays explaining computer music theory providing detailed analysis.
http://www.freewebs.com/gustavsphinx/

Schoen Musical Notation - Julius Schoens alternative to traditional musical notation, has music notation documents, reference, and discussion.
http://www.illuschoen.net/sn/sn.htm

Abacci Music Lessons - Music theory lessons with accompanying MIDI files.
http://www.abacci.com/music/lessons/default.asp

Visualizing Music: A Model of Harmony - Visual model of music which geometrically describes relationships in harmony.
http://www.geocities.com/yaaarrrgg/music/

Dolmetsch: Music Theory & History Online - Offers a musical dictionary, recorder lessons, instrument information and a composers listing.
http://www.dolmetsch.com/theoryintro.htm

The Musical Intervals Tutor - Offers interactive music intervals and self-testing.
http://www.musicalintervalstutor.com/

Fugue Treatises, Analyses and Tools - Bibliography of fugue analysis research, writings and analysis.
http://www.kunstderfuge.com/theory.htm

Bimodalism - A contemporary alternative to atonal styles of composition.
http://www.ubieta.com/bimodalism/

Music Theory - Covers intermediate and slightly advanced topics in tonal music theory.
http://www.fortunecity.com/tinpan/lennon/23/theory.html

Star Theory - Free preparatory syllabus in music theory and orchestration.
http://www.musicarrangers.com/star-theory/

The Musical Octave - Thomas Váczy Hightower's study of musical scales. Discuses the meta-physical properties of music and the creation of musical scales from an acoustic and mathematical perspective.
http://home22.inet.tele.dk/hightower/

Atonal Set Calculator - Interactive atonal set calculator for pitch class sets and twelve-tone rows. Enter one or two sets and find normal order, prime form, Forte number, interval vector, and symmetry.
http://ericandchar.com/atonal/

Analysis of Masterpieces - Analysis of full-music 20th Century atonal pieces.
http://www.geocities.jp/musikanalyse/eng/text/textE.html

Essentials of Music Theory - Summarizes entry-level music theory through advanced topics. Includes beginners' drills.
http://musictheory.redzeppelin.org/

Music Theory Society of New York State - Provides a forum for the exchange of information and to promote music theory as a scholarly and pedagogical discipline. Site also contains membership and scholarship information as well as open calls for papers.
http://www.ithaca.edu/music/mtsnys/

The Music Theory Minute - Online music theory tutorials for beginning students.
http://www.musictheoryminute.com/

A-Natural Atonality - Claims that atonality is unnatural while tonality is acoustically and historically natural.
http://www.greenwych.ca/atonal.htm

Nuottila - Online ear training fundamentals site. [Requires Java]
http://www.nuottila.info/2007/

Sight-Reading Rhythmic Patterns - Rhythmic exercises with accompanying MIDI files.
http://www.rhythm-patterns.narod.ru/

Music Theory Instruction - Online instruction for all musicians beginner or advanced. Covers scales, chordal theory, progression theory, modes and foreign scales.
http://www.angelfire.com/music5/theory/

Music Theory - A guide to music theory that including chords, scales, music notation and other music theory topics. Has weblog format; includes links as well.
http://www.musictheoryblog.blogspot.com

VCU Music Theory Resources - Music theory and ear training resources from Virginia Commonwealth University.
http://www.people.vcu.edu/~bhammel/theory/resources/index.html

Dynamic Spectrograms of Music - Provides a type of spectrogram suitable for understanding the structure of music.
http://nastechservices.com/Spectrograms.html

A Beginner's Guide to Modal Harmony - A concise explanation of the Gregorian and Renaissance modes and their development in the Common Practice era.
http://www.standingstones.com/modeharm.html

What is Music - Solving a Scientific Mystery - Provide information on the book by Philip Dorrell which explains a new scientific theory about music: the super-stimulus theory.
http://whatismusic.info/

Technical Committee on Musical Acoustics - A group within the Acoustical Society of America, that concerns itself with the application of science and technology to the field of music. Contains members, a list of papers, acousticians and links.
http://www.public.coe.edu/~jcotting/tcmu/

Theory on the Web - Intermediate music theory review for college students.
http://www.smu.edu/totw/toc.htm

The Ancient Musical Modes - Ideas regarding the "classical modes" described by Plato and Aristotle.
http://www.pathguy.com/modes.htm

Chordwizard: How Music Works - A concise summary of important concepts in music.
http://www.chordwizard.com/theory.html

Teoria - Includes software, books, exercises, and links.
http://www.teoria.com/

Auto-Transposer - Transposes all twelve major keys of chord progressions.
http://www.autotransposer.com/

Solomon's Music Resources - Resources for composers, music theorists, and researchers of music, with sound files, papers and compositions.
http://solomonsmusic.net/

Pattern Thinking in Music - Offers visual aid to recognizing musical patterns occurring in melody, harmony and rhythm. Provides online demonstration; requires download.
http://www.seepatts.com/

Gems of Compositional Wisdom - Articles on advanced atonal and serial concepts.
http://www.cosmoedu.net/DoctorFields/

Polytempo Music Articles - Articles by John Greschak. Includes an annotated bibliography of polytempo music.
http://www.greschak.com/polytempo/

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

Ricci Adams' Musictheory.net - Includes introductory and intermediate music theory lessons, ear trainers, and books.
http://www.musictheory.net/

Music Theory for the Short Attention Span - Essential music theory knowledge, briefly explained.
http://www.folkblues.com/theory/

Interval and Chord Ear-Training - This site drills ear-training (aural skills), focusing on the aural recognition of musical sounds, including intervals and chords.
http://www.auralskills.com/

SchenkerGuide - An introduction to Schenkerian analysis for undergraduate music students. Includes background, working method, glossary and bibliography.
http://www.schenkerguide.com/

Creativelab - Visual representations of music expressed in terms of color. In English, Ukrainian and Russian.
http://creativelab.kiev.ua/

Harmonic Bindings - A paper about the unification of Janecek's theory of imaginary tones with the two Risinger's principles of functional relations.
http://www.sweb.cz/vladimir_ladma/english/music/articles/ifsa97.htm

The Fugue - An outline of the fundamentals of a fugue based on Hugo Norden's "Foundation Studies in Fugue."
http://musik.freepage.de/cpb7079/inhalt.html