Just installed this on my iPhone and was impressed:
http://mister.aardvark.googlepages.com/iano

Video:
http://www.youtube.com/watch?v=ycIjt-ykWRo

It's a four octave piano that allows up to five keys to be pressed at once.
The capacitance touch screen means you don't have to press the keys very
hard.



--
Mike