Tink

I'll just quote the creator:

"Tink by William Pilgrim is an interactive musical toy for PCs inspired by Toshio Iwai's unique Tenori-on electronic instrument. Tink is free to download and can be interacted with in a variety of ways such as by head-tracker or one switch. ...


In essence Tink enables musicians to tinker with a 16 step sequencer that constantly plays and loops through the notes you place on a grid."
it's neat and it looks like it would run of a thumbdrive.

I found it from the accessible gamebase  which is "SpecialEffect is a charity dedicated to helping ALL young people with disabilities to enjoy computer games. We'll do whatever it takes, because we've seen first hand what a kick-start effect they can can have on motivation, inclusion and quality of life..." it looks like a neat place to find games and info for handicapped folks.