Mugician is free in the app store. Just go get it for iPad (iPad only).
Xstrument (ie: sometimes misspelled Xtrument)
This is a clone of the Samchillian instrument for a macbook laptop.
You can do some astonishing things on it with a very small number of keys to play with.
Xstrument on github
My Contact Info
rob.fielding@gmail.com - my primary email,fb,pp, etc.