Music Theory Online
http://www.societymusictheory.org/mto/
Published six times a year, Music Theory Online (MTO) is the electronic journal of the Society for Music Theory. Each issue includes a target article, book reviews, and reports from a distinguished panel of International Correspondents. In addition, MTO publishes announcements of upcoming conferences and calls for papers, a list of job opportunities, abstracts of recently completed and in-progress dissertations, and summaries of recently published books."

Ricci Adams Music Theory
http://www.musictheory.net/
Animated tutorial to music theory.

Harmony in Tonal Music
http://www.harmony.org.uk/
Guide to understanding the role of chord progressions in tonal music.

Society for Music Theory
http://www.societymusictheory.org/
Site for The Society of Music Theory journal, containing links to useful resources regarding music theory.

The Musical Intervals Tutor
http://www.musicalintervalstutor.info/
A study aid to instruct students on how to learn the sounds of the basic musical intervals from minor second to perfect octave.

ChordWizard: How Music Works
http://www.chordwizard.com/theory.html
Brief tutorial that covers the basics of music theory.