Chrome Music Lab - Song Maker, Shared Piano, Kandinsky, Voice Spinner, Rhythm and so much more! Lots of fun ways to explore music.
Drum Bit - create your own drum beat on a loop!
Play Xylo - Use your keyboard to play xylophone. Set up pitch names or solfege (do, re, mi...) notes!
Chord Chord - sequence chords with some cool drum beats. Great for writing songs!
Doctor Musik - TONS of fun games. Boom Writer, Robot Composer, Dragon's Choir, Staff Dungeon and virtual instruments.
San Fransico Symphony Kids - some interesting music activities including a fun composition maker!