What are some alternatives?
When comparing Piano Notes Fun and Synthesia, you can also consider the following products
Yousician - An interactive guitar-learning app.
Pianu - Learn to play the piano in your browser.
Simply Piano - Fast and fun way to learn piano
Flowkey - The easiest way to learn piano with your iPhone or iPad
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Playground Sessions - Online piano learning game.