What are some alternatives?
When comparing StepMania and Google Music Lab, you can also consider the following products
Frets on Fire - Bonus content.
Sampulator - Make (and record) beats on your keyboard
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.
Ramsophone - A generative art/music machine. (Be sure to refresh!)
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