Tonic Tutor: My favorite web-based music games
PBS Kids Music Games
Making Music Fun Arcade: Scroll down for the games
San Francisco Symphony: Fun & Games with Music
Whack a Note: Music note flashcard game
Simply Piano (iOS) Teach yourself to play piano!
Piano Maestro (iOS) Learn to play along to backing tracks; Beginner to Late Intermediate Levels.
My Note Games (iOS) Note reading & ear training games for lots of different instruments
Rhythm Cat (Android & iOS) Super fun rhythm game!
Treble Cat & Bass Cat (Android & iOS) More note reading games
NoteStar (iOS) Play pop songs on the piano along with a full band
iReal Pro (Android, iOS) Play along to blues and jazz backing tracks
Ningenius (iOS) Note reading challenge for almost any instrument
Making Music Fun: Free sheet music for many instruments
G Major Music Theory: Free guitar and piano sheet music
Heartwood Guitar: Free guitar tabs with great tutorials
Blank Sheet Music: Download and print manuscript paper
Musicnotes.com: Download your favorite songs arranged for multiple skill levels and instruments (Check out the Free Stuff and Easy Pop sections)
Noteflight: Create a new composition! (Current students log in at Songwriting for Kids Noteflight)