Also refer to the "Our Bands" section for grade-level-specific resources/sites!
Tonal Energy Tuner - All-in-one tuner AND metronome app. Produces drone pitches to help match/internalize sound. "Smiles" when you're in tune!
Pro Metronome - A very easy-to-use metronome app.
Bandmate Tuner - Displays written note on staff. Transposes for your instrument.
Music Dictionary - Look up any music term. Provides definition and pronunciation.
Classics for Kids - A very interactive site that lets you explore a variety of subjects, primarily music history periods/composers.