Metronome Online - Free online metronome
Note Flight - Great site for notating music. You can create a free account.
Soundation - Audio workstation where you can record yourself, mix tracks, and create loops. You can create a free account.
Making Music Fun - Great site for beginners including free printable worksheets and sheet music for a variety of instruments.
Resources for each instrument, including free sheet music and exercises:
Music Theory - Helpful site to learn music theory and practice with exercises.
Teoria - Helpful site to practice ear training
8notes - Free sheet music resource