What are some alternatives?
When comparing Flowkey and Synthesia, you can also consider the following products
Simply Piano - Fast and fun way to learn piano
Playground Sessions - Online piano learning game.
Pianu - Learn to play the piano in your browser.
Skoove - Babbel for learning the piano
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Yousician - An interactive guitar-learning app.