What are some alternatives?
When comparing Incredibox and Frets on Fire, you can also consider the following products
Audiosurf - Audiosurf is a music-adapting puzzle racer where you use your own music to create your own experience.
StepMania - Rhythm game for Windows, Mac, and Linux.
Rocksmith - Rocksmith builds off of the fundamentals of music games like Rock Band and Guitar Hero by offering fundamental lessons in actually playing guitar.
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Splice Beat Maker - Make and share beats in your browser
Ramsophone - A generative art/music machine. (Be sure to refresh!)