Mike Mangini's Rhythm Knowledge Online
It offers rhythm instruction materials for all instruments.
http://www.rhythmknowledge.com/

Dr Downing Music
Troubleshooting books for all musicians.
http://www.drdowningmusic.com

ADG Productions
Books, CDs, videos and software for most instruments and styles.
http://www.adgproductions.com/

Southern Music Company
Sheet music and methods for all instruments and ensembles.
http://www.southernmusic.com/

Musician's Workshop
Video and audio products for string and keyboard instruments, harmonica, and tinwhistle.
http://www.musicians-workshop.com/

Berklee Press
Books and recordings covering all instruments and all main music topics.
http://www.berkleepress.com/

Edly's Musical EdVentures
Sells two books: one for to learn music theory and one for piano.
http://www.edly.com/

Music Minus One
Participatory recordings and accompaniment music for musicians and vocalists, including all styles.
http://www.musicminusone.com/

MusickEd.com
Study guides and learning tools for all instruments. Also online lessons.
http://musicked.com/

Cassette and Video Learning Systems
Instructional books, DVDs and video for guitar, bass, keyboards, drums, harmonica, banjo mandolin, violin and fiddle.
http://www.cvls.com/

Music in Motion
Sells books, videos, audios, awards, teaching aids, posters, gifts and software.
http://www.musicmotion.com/

Comprehensive Ear Training
A progressively graded series of books, CDs, cassettes, and MIDI discs.
http://www.keystrokepublishing.com/

Healing Music
Books, CDs and tapes for to practice singing and songwriting.
http://www.healingmusic.net/

Allstate Sight Reading
Books for all instruments and choral students.
http://home.att.net/~l.mclure/

Flying Leap Music
Publishers of You've Got Rhythm, a method to learn and improve music-reading skills.
http://www.fleap.com/

Creative Horizons Music
Sells biblical psalms sheet music, a chord scale chart, and a handbook explaining music theory.
http://www.creativehorizonsmusic.com/

Comp-U-Xpress, Inc.
Offers songbooks and instructional books, CDs, and DVDs for various instruments and styles.
http://www.easymusicbooks.com/

Allegro Music Online
Complete line of products including popular piano methods and Suzuki materials.
http://www.allegromusiconline.com/

Pro Audio Music
Wide selection of books, videos, CDs and software for all instruments and music topics.
http://proaudiomusic.com/educational.htm

MVP: Music
Offers an extensive collection of instructional videos.
http://www.mvphomevideo.com/

One Day at a Time
Offers a selection of musical instructional products for all instruments and styles.
http://www.1dayx.com/

Music Message International
Offering instructional videos for several instruments.
http://www.musicmessage.com/

ModRock.com
Guitar tabs, books, videos, sheet music, and CDs for all instruments.
http://www.modrock.com/

Musicridge
Offers instructional videos for accordion, stringed instruments, drums, and brass ensembles.
http://www.musicridge.com/

Ron Greene's Music Dials
Visual reference guides that show the chords and scales you can play with all instruments.
http://www.musicdials.com/

Musicroom.com
Music tuition and music-related books for all instruments.
http://www.musicroom.com/mfschools.ihtml

Letsplaymusic.com
Offering several educational books for accordion, harmonica, and guitar.
http://www.letsplaymusic.com/

Perfect Pitch
Offers a training program by David Lucas Burge, designed to teach pitch recognition by ear. Includes research information, and a partial lesson.
http://www.perfectpitch.com