Musicnotes sells over 300,000 sheet music arrangements and has been providing these for over 20 years.
Flowkey is an online tool for practicing piano or keyboard with an interactive interface.
Gear4music sells guitars, drums, keyboards, and more music gear. Their mission is to make purchasing and owning instruments and equipment easy through good prices and good service.
Learn and Master by Legacy Learning Systems offers educational courses for guitar, piano, drums, painting, dance, and photography.
Rocket Piano offers piano playing lessons, from beginner to advanced. Signing up for their courses all gets you a lot of freebies.