The Woodwind Fingering Guide (Fingering charts for Flute, Clarinet, Saxophone, Oboe, and Bassoon)
Horn Fingering Chart (Top fingerings for single horn, bottom are for double horn)
Baritone/Euphonium Fingering Chart
Just For Brass (Online resources for all brass instruments)
Vic Firth (Check out 'education' section for help on rudiments, mallet techniques, and much more)
Drum Corps International (Marching music's major league)
National Association for Music Education
PA Music Educators Association
musictheory.net (online music theory lessons and quizzes)
Teoria (music theory and ear training lessons)